Catalysed Hydrothermal Carbonization of Woody Biomass

and Recycling of Process Liquid

by

Amin Ghaziaskar

A thesis submitted to

the Faculty of Graduate and Postdoctoral Affairs

in partial fulfillment of the requirements for the degree of

Master of Applied Science

in

Mechanical Engineering

Ottawa-Carleton Institute for Mechanical and Aerospace

Engineering

Carleton University

Ottawa, Ontario

© 2018, Amin Ghaziaskar

i

Abstract

The research introduces catalysed hydrothermal carbonization (CHTC) as a

treatment method for woody biomass to produce solid hydrochar as well as

value-added compounds (VACs). The hydrochar had higher heating values

(HHV) of 28.3 MJ/kg and H/C and O/C ratios similar to coal, when using the

liquid:biomass (L:B) ratio of 12:1 for the process. Densified CHTC hydrochar

pellets were 97% durable, and hydrophobic when compared with wood pellets

and torrefied wood pellets. CHTC has the potential to produce VACs such as

glycolic acid, formic acid, acetic acid, levulinic acid, 5-hydroxymethylfurfural,

and furfural, from the process liquid effluent. Recycling the process liquid at

L:B of 5:1 increased the concentration of the VACs and energy yield of the

hydrochar. The production yields of the VACs were dependent upon the L:B

ratios, they increased parallel with L:B ratios.

ii

Preface

The following thesis is a combination of two separate journal articles. I was

fully involved in setting up and conducting the research, obtaining data and

analyzing results, as well as preparing and writing the material presented in

the co-authored articles integrated into this thesis.

iii

Acknowledgements

Glenn A. McRae

Onita D. Basu

Edward P. Lai

Alexis Mackintosh

Maia P. Gill

Kafeel Kamal

Tony O’Neil

Geoff Seater

Marie Tudoret

Guy Tourigny

Margaret A. Knowling

Neelesh Bhadwal

Airex Energy

iv

Table of Contents

Abstract ............................................................................................................................... i

Preface ................................................................................................................................ ii

Acknowledgements .......................................................................................................... iii

Table of Contents ............................................................................................................. iv

List of Tables .................................................................................................................. viii

List of Figures ................................................................................................................... ix

Chapter 1: Introduction ................................................................................................... 1

1.1 Research Objectives .............................................................................................................. 3

1.2 Thesis Structure ..................................................................................................................... 4

1.3 Article Summary.................................................................................................................... 6

Chapter 2: Literature Review .......................................................................................... 8

2.1 Biomass ............................................................................................................................... 16

2.2 Biomass Conversion Technologies ..................................................................................... 18

2.3 Hydrothermal Carbonization (HTC) ................................................................................... 23

2.3.1 Underlying Reactions ................................................................................................... 24

2.3.2 HTC Products ............................................................................................................... 27

2.3.2.1 Solid ...................................................................................................................... 28

2.3.2.2 Liquid .................................................................................................................... 35

2.3.2.3 Gas ........................................................................................................................ 36

2.3.3 Temperature ................................................................................................................. 36

2.3.4 Residence Time ............................................................................................................ 37

2.3.5 Pressure ........................................................................................................................ 37

v

2.3.6 Solid Load .................................................................................................................... 38

2.3.7 pH ................................................................................................................................. 39

2.4 Catalysed Hydrothermal Carbonization (CHTC) ................................................................ 39

Chapter 3: Catalysed Hydrothermal Carbonization with Process Liquid Recycling

........................................................................................................................................... 41

3.1 Introduction ......................................................................................................................... 43

3.2 Methods and Materials ........................................................................................................ 46

3.2.1 Catalysed Hydrothermal Carbonization (CHTC) ......................................................... 46

3.2.2 Materials ....................................................................................................................... 47

3.2.3 Characterisation of the Solid Hydrochar ...................................................................... 48

3.2.4 Densification ................................................................................................................ 50

3.2.5 Mechanical Strength of Hydrochar Pellets ................................................................... 50

3.2.5.1 Durability Test ...................................................................................................... 50

3.2.5.2 Compression Test ................................................................................................. 51

3.2.5.3 Electrochemical water ingress (EWI) test ............................................................. 52

3.3 Results and Discussion ........................................................................................................ 52

3.3.1 Characterisation of the Solid Hydrochar ...................................................................... 54

3.3.1.1 Influence of Process Liquid Recycling on Atomic Ratios (H/C-O/C) of the

Hydrochar ......................................................................................................................... 54

3.3.1.2 IR Spectroscopy of the Hydrochar ........................................................................ 57

3.3.1.3 SEM Images of the Hydrochar ............................................................................. 61

3.3.1.4 Mass Yields and Energy Values ........................................................................... 63

3.3.2 Densification ................................................................................................................ 65

3.3.3 Mechanical Properties of Hydrochar Pellets ................................................................ 66

3.3.3.1 Durability Test ...................................................................................................... 67

vi

3.3.3.2 Compression Test ................................................................................................. 68

3.3.3.3 Electrochemical water ingress (EWI) test ............................................................. 69

3.4 Conclusion ........................................................................................................................... 73

3.5 Acknowledgements ............................................................................................................. 74

Chapter 4: Production of Organic Compounds through Recycling the Process Liquid

of Catalysed Hydrothermal Carbonization .................................................................. 75

4.1 Introduction ......................................................................................................................... 78

4.2 Methods and Materials ........................................................................................................ 81

4.2.1 Catalysed Hydrothermal Carbonization (CHTC) ......................................................... 81

4.2.2 Materials ....................................................................................................................... 82

4.2.3 Characterization of the Process Liquid ........................................................................ 83

4.2.4 Characterization of the Solid Hydrochar ...................................................................... 85

4.3 Results and Discussion ........................................................................................................ 86

4.3.1 Recycling the Process Liquid ....................................................................................... 86

4.3.2 Effects of L:B Ratio on the VAC Concentrations and Hydrochar ............................... 93

4.4 Conclusion ........................................................................................................................... 97

4.5 Acknowledgments ............................................................................................................... 97

Chapter 5: Challenges and the Future .......................................................................... 98

Chapter 6: Conclusions ................................................................................................ 100

Chapter 7: References .................................................................................................. 104

Appendices ..................................................................................................................... 119

Appendix A : Contribution of the Authors ............................................................................. 119

Appendix B : Hydrothermal Carbonization of Woody Biomass ............................................ 121

Appendix C : Instrumentation ................................................................................................ 124

Appendix D : Supplemental Data to Chapter 4 ...................................................................... 130

vii

D.1 IR Spectroscopy ..................................................................................................... 130

D.2 SEM Images of the Hydrochar and the Biomass ................................................... 134

Appendix E : UFLC Calibration Curves ................................................................................ 142

viii

List of Tables

Table 2-1: Functional groups found in the biomass with their corresponding wavenumber.

Data gathered by Reza et al. [55]. ..................................................................................... 30

Table 3-1 Elemental composition (on dry basis, % mass) of the biomass and hydrochar,

energy content, energy densification ratio, and energy yield, (uncertainties in

measurements are in units of least significant figures unless otherwise noted). 1 ............ 56

Table 3-2 Durability of the hydrochar pellets with pressure. Durability estimates were

similar for pellets made with pressures ≥ 135 MPa, but dropped by ≈2% when the pressure

was reduced to 70 MPa. .................................................................................................... 68

Table 3-3: Compressive strength of densified hydrochar pellets with densification pressure.

The densification pressure was found to affect the compressive strength of the pellets while

recycling the process liquid did not. ................................................................................. 69

Table 4-1 Higher heating value (HHV) and compositional analysis of the woody biomass

on a dry basis. ................................................................................................................... 83

Table 4-2 Mass yield, HHV, energy densification ratio, and compositional analysis (weight

percentages on dry basis) of the hydrochar after recycling the process liquid 15 times with

L:B ratio of 5:1.................................................................................................................. 90

Table 4-3 Compositional analysis (dry basis weight percentages) of the hydrochar made

with different L:B ratios through CHTC process. ............................................................ 95

ix

List of Figures

Figure 2-1: Fast carbon cycle in between land, atmosphere and the ocean. The values are

in gigatons of carbon per year. Yellow coloured numbers show neutral fluxes of carbon,

while white coloured numbers show the stored carbon. The red coloured numbers show

human contributions to the cycle every year [13]. .............................................................. 9

Figure 2-2: Total anthropogenic GHG emissions (GtCO2eq / yr) by economic sectors in

2010, which totals to 49 GtCO2 eq/yr. AFOLU includes land-based CO2 emissions from

forest fires, peat fires and peat decay. ............................................................................... 11

Figure 2-3: Global energy-related CO2 emissions [16]. The data from international energy

agency (IEA), shown in the figure, include all the energy-related CO2 emissions from the

five economic sectors used by IPCC. ............................................................................... 12

Figure 2-4: Different biomass conversion technologies, specifically thermochemical

conversions. The processes are connected to the product forms each produce.

Carbonization, liquefaction, and gasification can be both thermal and hydrothermal. .... 19

Figure 2-5: Different hydrothermal processes separated according to their required

temperature range on a water phase diagram. The major products of each process are

written in parentheses. The figure is an adaptation from the work of Kambo et al. [42]. 22

Figure 2-6: Products of hydrothermal carbonization (HTC) separated according to their

phases (i.e., gas, liquid, hydrochar solid) [1]. ................................................................... 23

Figure 2-7: Hydrolysis of cellulose into oligosaccharides and glucose components [49].

........................................................................................................................................... 25

Figure 2-8: Chemical pathways for biomass components after decomposition [50]. ...... 27

x

Figure 2-9: Products and possible applications of HTC [53]. ......................................... 28

Figure 2-10: Infrared spectra of initial biomass and final hydrochar after treatment at 240

°C. ..................................................................................................................................... 29

Figure 2-11: Van Krevelen diagram of HTC hydrochar produced with different conditions

and from different biomass types [42]. ............................................................................. 31

Figure 3-1: Percentage ratio of make-up catalyst solution (Catalyst) to recovered process

liquid (Process liquid) used in 15 recycle experiments (stacked columns), circles show pH

of the liquid biomass mixture prior to start of the process (initial pH), and diamonds show

pH of the process liquid after CHTC (final pH). .............................................................. 53

Figure 3-2: Van Krevelen diagram showing atomic ratios of the feedstock biomass and the

hydrochar samples. The dehydration and decarboxylation reactions are enhanced by the

catalyst producing CHTC hydrochar with H/C and O/C ratios that fall in the region where

lignite and coal overlap. The H/C and O/C ratios of the hydrochar do not vary significantly

with recycling. The hydrochar produced without catalyst (HTC), but otherwise identical

processing conditions, falls within the lignite region of the diagram, shown by the blue

square. ............................................................................................................................... 57

Figure 3-3: IR spectra of raw biomass, Fresh, and R15. The spectra of the hydrochars were

similar, but different when compared with the spectra of the feedstock biomass. ........... 60

Figure 3-4: SEM images: (a, b) feedstock biomass showing fibrous structure; (c, d)

spherical structure of the hydrochar from fresh catalyst solution (Fresh); (e, f) porous

structure of the hydrochar R1; (g, h) R15 showing spherical and porous structure. The

xi

images of the hydrochar were indistinguishable, but different from the feedstock biomass.

........................................................................................................................................... 62

Figure 3-5: Higher Heating Value (HHV), mass and energy yield of the hydrochar

samples. The mass yield increased by 2.7 percentage points for R1 compared with the

initial process, which was done using only the catalyst solution as the starting liquid phase

(Fresh). Energy yield increased from 64% (Fresh) to an average of (68.6 ± 1)% for R1 to

R15. The HHV is essentially unchanged for all the recycles. .......................................... 63

Figure 3-6: Average density and energy density of pellets made at four different pressures.

The density did not change after increasing the compression from 135 MPa to 155 MPa.

The energy density reached 30 GJ/m3 at a density of 1070 ± 30 kg/m3. .......................... 66

Figure 3-7: Current through an electrochemical cell with pellet electrodes made of

hydrochar, wood, and torrefied wood. The wood disintegrated after a few minutes.

Torrefied wood increased 42% in mass after 24 h in the electrolyte. The small currents

observed for hydrochar pellets suggests much less water ingress. ................................... 72

Figure 3-8: Pictures of the pellets used in the EWI tests when they were first immersed in

the electrolyte (time=0) and later at the times indicated. The hydrochar pellets showed no

visible indications of breakdown products in the electrolyte, they maintained their shape

and passed low currents throughout the tests, as shown in Figure 3-7. ............................ 72

Figure 4-1: (a) VAC concentrations and (b) HHV, mass yield, and energy yield of the

hydrochar produced after recycling the CHTC process liquid, at 5:1 L:B ratio. .............. 87

Figure 4-2: The sum of AA, GA, and LA concentrations, represented as data and the fitted

model................................................................................................................................. 89

xii

Figure 4-3: (a) Concentration of VACs in the process liquid after using different L:B ratios

with CHTC and (b) production yield of the VACs with respect to dry weight of the biomass

used. .................................................................................................................................. 94

1

Chapter 1: Introduction

Biomass is a renewable source of energy, which can supplement or at

best replace the fossil fuels such as coal. However, this task can be challenging

depending on the properties of the biomass e.g., moisture content, energy

value, uniformity, biological conditions.

Different treatment methods and processes are currently being used to

convert biomass into a more stable source of energy to prevent further

decomposition of the biomass, which results in the release of green house gases

(GHGs) such as carbon dioxide (CO2) and methane (CH4) into the atmosphere.

Hydrothermal carbonization (HTC), as one of the methods that convert

biomass into a more energy dense fuel (i.e., hydrochar) has recently been

getting attention from researchers, environmentalists, and the industries that

use coal as their source of energy. This process uses water to breakdown the

chemical structure of the biomass through hydrolysis; therefore, it can treat

the biomass that is already wet and can decompose into GHGs.

HTC has the potential to produce a wide range of products in the form

of gas, liquid, and solid. Although the main product of the process is recognized

as hydrochar in the form of a solid, the output gases and the process liquid can

be used to maximize its productivity. The gases produced throughout the

2

process include H2, CO2, and CH4, while the process liquid carries various

valuable organic compounds such as furfural, 5-hydroxymethylfurfural, and

levulinic acid. These compounds can potentially be harvested in order to add

value to the process.

Several operating conditions of the HTC process, such as temperature,

time, and liquid:biomass ratio, have been found to affect the production yields

and characteristics of the hydrochar, gases, and other organic compounds in

the liquid [1]. The addition of acidic and basic compounds to the process liquid

(pH adjustment) has been shown to improve the quality of the hydrochar, e.g.,

higher heating value (HHV) and increased carbon content [1]–[3].

Catalysed hydrothermal carbonization (CHTC) of biomass is similar to

HTC except for the use of an acidic catalyst, which is added to the process

liquid [4]. This process can potentially produce a hydrochar with increased

HHVs and concentrations of organic compounds in the process liquid after

biomass treatment compared with HTC.

One of the challenges in operating the HTC and CHTC processes is the

use of water, which can negatively affect the sustainability of the process. To

overcome this challenge, recycling the process liquid has been performed for

the HTC process [5]. Recycling the process liquid increases the mass yield,

3

improves the HHV of the hydrochar, and changes the concentrations of organic

compounds in the process liquid [5]–[8].

Consideration of operating time and temperature of CHTC is also at

great importance; however, studying the possibilities of recycling of process

liquid and changing the volume of the liquid used for the process can be more

advantageous. The main portion of the CHTC system, which requires the most

heat for temperature increase, has been identified to be water. Increasing the

temperature from approximately 24 °C to boiling temperatures of 100 °C is

also more energy intensive compared with keeping the operation near

temperatures of 200-300 °C. Lowering the overall volume of the liquid used in

the process and hot recycling of process liquid can cause tremendous energy

savings regardless of what the operating time and temperatures are.

1.1 Research Objectives

This thesis mainly studies the effects of recycling the proprietary

catalyst used for the CHTC process on a wide range of parameters such as the

mass yield, energy content, elemental composition, compressibility of the

hydrochar, water absorptivity of the pellets made from the hydrochar, and

composition of the process liquid (effluent). In addition, the liquid:biomass

ratio used for CHTC is optimized in order to decrease the amount of energy

4

used for the CHTC process while preventing any loss in the quality of the

hydrochar.

The objectives of this thesis can be summarized into the following:

I. Study the effects of partial recycling of the CHTC process liquid

on the physical characteristics of the hydrochar.

II. Study the effects of full recycling of the CHTC process liquid on

the chemical characteristics of the liquid effluent.

III. Study the effects of the liquid:biomass ratio on the hydrochar and

process liquid effluent of the CHTC process.

1.2 Thesis Structure

Chapter 1: an introduction to the HTC of biomass, CHTC, and defining

the objectives of the thesis

Chapter 2: a review on the research done in the field of biomass

treatment, specifically HTC

Chapter 3 (Submitted, under revision): introduces CHTC of woody

biomass, while focusing on partially recycling the process liquid used for CHTC

5

processes. Furthermore, the physical-chemical properties of the hydrochar are

presented and discussed.

Chapter 4 (Planned submission): presents full recycling of the CHTC

process liquid while studying its effects on the hydrochar and the concentration

of some Value-Added Compounds (VACs). Discusses the effects of the

liquid:biomass ratio on the concentration of the VACs and the properties of the

hydrochar.

Chapter 5: presents challenges to further study of the CHTC process

and future work needed to be done.

Chapter 6: summarizes the research work with a focus on the

significant conclusions.

Appendix A: presents the contributions of the authors to the articles

Appendix B: presents the results of a set of controlled experiments done

to compare the HTC process with the CHTC, which includes the properties of

the hydrochar and the process liquid.

Appendix C: presents images of the instruments used throughout this

research.

6

Appendix D: presents the supplemental data to Chapter 4.

Appendix E: presents the calibration curves used for determining the

chemical concentrations in the process liquid using Ultra Fast Liquid

Chromatography (UFLC).

1.3 Article Summary

Article 1: Catalysed Hydrothermal Carbonization with Process Liquid

Recycling

Authors: Amin Ghaziaskar1, Glenn A. McRae1, Alexis Mackintosh2, Onita D.

Basu3

Submitted to: Journal of Energy and Fuels

1Department of Mechanical and Aerospace Engineering, Carleton University,

Canada, K1S 5B6

2PCS Technologies Inc., Vancouver, Canada

3Department of Civil and Environmental Engineering, Carleton University,

Canada, K1S 5B6

7

Article 2: Production of Organic Compounds Through Recycling the Process

Liquid of Catalysed Hydrothermal Carbonization of Lignocellulosic Biomass

Authors: Amin Ghaziaskar1, Glenn A. McRae1, Alexis Mackintosh2, Edward

P.C. Lai3, Onita Basu4

Planned submission to: Journal of Bioresource Technology

1Department of Mechanical and Aerospace Engineering, Carleton University,

Canada, K1S 5B6

2PCS Technologies Inc., Vancouver, Canada

3Department of Chemistry, Carleton University, Canada, K1S 5B6

4Department of Civil and Environmental Engineering, Carleton University,

Canada, K1S 5B6

8

Chapter 2: Literature Review

Climate change has significantly impacted our surrounding

environment. Glaciers are melting, which has resulted in the rise of sea level.

Drought in different parts of the world such as South Africa, the middle east,

and USA have been more prevalent [9]. Recent years have been the warmest

on record with the years 2016, 2015, and 2017 being the 1st, 2nd, and 3rd,

respectively, while it is speculated that 2018 will be the 4th [10]. Life-

threatening natural events such as hurricanes and tsunamis have become

frequent for regions across the world. The role of human beings in climate

change is evident; a group of 1300 independent scientists from all over the

world have concluded that there is a 95% probability that human activities

over only the last 50 years have increased the overall temperature of the planet

[11]. In context, our industrial activities have increased the atmospheric

carbon dioxide levels from 280 parts per million to over 400 parts per million

in the last 150 years [11].

Climate change has caused fire seasons in Canada to start earlier, and

to last longer. Extreme forest fires have been occurring in Canada in the past

few years. On average, Canada faces 7500 fires a year, which on average have

burned 2.4 million ha per year, since 1990 [12]. However, 2015, with 7140 fire

incidents that burned 3.9 million ha, was the third year in a row with above

9

average area burned [12]. Furthermore, large fires with extreme consequences

can happen even in years with lower averages of fire occurrences.

Figure 2-1 illustrates the neutral carbon cycle of Earth and the negative

impact of human activities on it by using the carbon that has taken millions of

years for nature to deposit into reservoirs such as fossil fuels [13].

Figure 2-1: Fast carbon cycle in between land, atmosphere and the ocean. The values are in gigatons

of carbon per year. Yellow coloured numbers show neutral fluxes of carbon, while white coloured

numbers show the stored carbon. The red coloured numbers show human contributions to the cycle

every year [13].

10

The flow of carbon within the atmosphere from one reservoir to another

is called the carbon cycle. The fluxes within this cycle can be slow (the buildup

of sediments in oceans, or formation of fossil fuels) or fast. Human activities

contribute to the fast carbon cycle by removing from the vast underground

hydrocarbon reserves and adding it to the atmosphere, as shown in Figure 2-1

[13]. Total anthropogenic greenhouse gas (GHG) emissions (e.g., carbon

dioxide (CO2), methane (CH4), nitrous oxide (N2O), perfluorocarbons (PFCs),

hydrofluorocarbons (HFCs), sulphur hexafluoride (SF6) and nitrogen

trifluoride (NF3)), were rising continually at an average rate of 1.3% (0.4

GtCO2eq) per year from 1970 to 2000, which then went up to 2.2% (1 GtCO2eq)

per year for the years 2000 to 2010. The total GHG emissions were measured

at 49 GtCO2 eq/yr for the years 2000 to 2010, the highest in human history [14,

15].

Human activities which contribute to the production of GHG emissions,

separated by economic sector, using the same designation as the

intergovernmental panel on climate change (IPCC), are 1-Energy; 2-

Agriculture, Forestry and Other Land Use (AFOLU); 3-Industry; 4-Transport;

and 5-Buildings, which make up, 34.6%, 24%, 21%, 14%, and 6.4%,

respectively, of the 49 GtCO2 eq/yr [14], as shown in Figure 2-2. Figure 2-3

shows the global energy-related CO2 emissions, which have been rising since

11

the year 2000 [16]; therefore, developing proper mitigation plans against

further CO2 emissions from this sector is vital.

Figure 2-2: Total anthropogenic GHG emissions (GtCO2eq / yr) by economic sectors in 2010, which

totals to 49 GtCO2 eq/yr. AFOLU includes land-based CO2 emissions from forest fires, peat fires and

peat decay.

In the last decade, growing energy demands and the increased share of

coal in the global fuel mix have caused this increase in GHG emissions [11].

Decarbonizing happens more rapidly in the energy sector compared with the

industry sector [14].

12

Figure 2-3: Global energy-related CO2 emissions [16]. The data from international energy agency

(IEA), shown in the figure, include all the energy-related CO2 emissions from the five economic sectors

used by IPCC.

The use of low carbon electricity supply technologies (i.e., renewable

energy, nuclear, and carbon capture and storage (CCS)) should increase from

the current 30% (in 2014) share of the energy sector to 80% by 2050, and to

100% by 2100 according to the IPCC mitigation plans developed in 2014 [14].

In addition, according to the international energy agency’s (IEA) sustainable

development scenario, the global emissions need to peak soon and decline

rapidly into 2020, which requires the share of low-carbon energy sources to

increase by 1.1% every year, which is more than five times the growth

13

registered in 2017 [16]. Even after meeting the IEA’s scenario requirements,

the temperature increase has been estimated to be 1.7-1.8 °C by 2100, which

still depends on the rate of global CO2 emission reductions [17].

The transition from using high carbon energy sources, such as coal and

oil, to renewable energy, nuclear, and CCS should be well planned and done

wisely. In 2005, the province of Ontario started to phase out all coal-fired

power plants. This effort was based on a cost-benefit analysis, which suggested

$3 billion annual savings in health care expenses as a result of lowering smog-

related air contaminants [18]. However, McKitrick and Aliakbari found that

the effects of closing the power plants resulted in only small improvements in

the air quality of some locations in Ontario, while the effects of coal power

plants could have been nullified by installing new pollution control systems

near the power plants.

Coal held the largest share (38%) in global power generation in 2017,

the same as in 1998, despite the efforts encouraging the shift from coal to other

sources of energy [19]. The share of oil and gas was 27% while the non-fossil

fuels had a share of 35% in the global power generation sector in 2017 [19].

The production of coal in the world was reported at 7.3 billion tonnes in

2017 after peaking at 8.1 billion tonnes in 2013, while it was 6.4 billion tonnes

in 2007 [20]. Total coal consumption in China alone reached 3.8 billion tonnes

14

in 2017, more than 60% of its total energy mix, which is more than half of the

coal produced in the world [21]. Canada produced 61 million tonnes (MT) of

coal in 2017, out of which 56% was metallurgical (used for making steel), and

44% was thermal coal (used for electricity) [20]. Furthermore, 9% of the energy

generated in Canada uses coal, which totalled 34.3 MT in 2016 [20].

Meanwhile, the government of Canada has planned to phase out coal-burning

power plants by 2030, which would eliminate the use of thermal coal but not

metallurgical coal.

In Canada, 17.4% of the total energy supply was renewable in 2016,

67.5% (2055 petajoules (PJ)) of which came from moving water; other

technologies, such as wind, solar, and biomass were also used [20]. After

hydropower, biomass had the biggest cut of the total renewable energies used

in Canada at 22.6% [20].

Canada has vast amounts of biomass in the form of forestry (998 M ha)

and standing biomass carbon stock, which has been estimated at 15835 MT

with a 5%-10% increase every year [22]. The extractable energy from these

resources is equal to 566 exajoules (EJ), which is the equivalent of 69 years of

Canada’s energy demand, currently supplied by fossil fuels. In addition,

Canada’s annual biomass produced from the forestry and agricultural divisions

is about 143 MT C per year, which is equal to 5.1 EJ (equivalent to 62% of

15

energy derived from fossil fuels per year) [22]. Sixty megaton of this total

amount is the annual residual or waste biomass, enough to produce 1.5 EJ of

energy every year; however, these resources are widely left untouched to decay

over time due to lack of technology and knowledge.

Biomass makes up one-third of the total energy consumption of

developing countries in Asia and Africa, mostly because of its availability

compared with fossil fuels such as coal and natural gas [23]. In countries such

as India, with vast reserves of coal (250 billion tonnes) in only specific regions,

the supply of the fuel to the whole country would not be financially or even

physically possible, while biomass is more evenly distributed over the country

and generally available [23]. It is essential to find, develop, and optimise

technologies, which can make use of this vast energy resource on the planet.

16

2.1 Biomass

“Biomass is the plant material derived from the reaction between CO2

in the air, water, and sunlight via photosynthesis, to produce carbohydrates

that form its building blocks” [24].

Biomass has been characterised differently by researchers. However, one

method is to put biomass into four different categories [24]:

• Woody plants

• Herbaceous plants/grasses

• Aquatic plants and algae

• Manures

Biomass consists of cellulose, hemicellulose, lignin, and small amounts

of extractables [24], [25]. Algal biomass and aquatic plants are mainly made

up of lipids, carbohydrates and proteins [26]. Cellulose and hemicellulose,

carbohydrates, are the primary load-bearing elements of the plant while lignin,

provides stiffness and rigidity [27]. Therefore, trees (lignin content of 20% to

30%) can grow taller before they bend compared with grass (lignin content of

less than 20%) [28].

17

Cellulose has been recognized as the most abundant natural polymer

with an estimated annual worldwide production of 1.5 teratonnes and it is

considered to be a nearly inexhaustible source of raw biomass material [29].

Cellulose is generally represented by the chemical formula (C6H10O5)n, which

is a long chain of polysaccharides. The average molecular weight of cellulose

is around 100,000 g/mol [24]. Cellulose is generally the most substantial

fraction of biomass, 40-50% by weight [24].

Hemicellulose forms approximately 20-30% of the dry weight of many

wood species and is an amorphous heteropolysaccharide. The structure of

hemicellulose is also highly dependent upon the biomass material [26], [30].

Hemicellulose is almost entirely composed of sugars such as glucose, mannose,

xylose, and arabinose and other five-carbon monosaccharides, with an average

molecular weight of <30,000 g/mol [24].

Lignin is a natural polymer in which aromatic compounds consist of

phenylpropane units, with hydroxyl and methoxyl groups connected mainly

through ether bonds [31]. Lignin is an amorphous material, which is

hydrophobic and has low solubility in water. Hardwood is made of 18-25%

lignin, while softwood has a higher lignin content of 25-35% [32]. Lignin is

used to help strengthen the structure of the plants [31], [32]. Lignin has

18

around 30% higher energy content (22-29 MJ/kg) compared with cellulose and

hemicellulose [33]–[35].

Biomass can be distinguished by its energy, moisture, fixed carbon,

volatile matter, ash, and alkali metal contents, as well as its cellulose/lignin

ratio [24]. Higher energy contents, lower ash and moisture contents, as well

as lower alkali contents, are often preferred for biomass to be used as a fuel

source.

2.2 Biomass Conversion Technologies

Biomass conversion technologies include thermochemical and

biochemical conversions, as shown in Figure 2-4. Thermochemical conversions

are not only more rapid than biochemical conversions but they also convert the

biomass into a more energy dense fuel by heating it under a pressurized and

oxygen deprived enclosure [36]. Biochemical conversions occur naturally

through aerobic and anaerobic degradations, fermentation, and enzymatic

hydrolysis [26]. Thermochemical conversion can eliminate pathogens and

potentially organic contaminants such as pharmaceutically active compounds

that might be in the feedstock [37]. Moreover, valuable solid, liquid, and gas

products can be produced through thermochemical conversions [37].

19

Thermochemical conversions include gasification, pyrolysis, combustion, and

hydrothermal processes, as shown in Figure 2-4.

Figure 2-4: Different biomass conversion technologies, specifically thermochemical conversions. The

processes are connected to the product forms each produce. Carbonization, liquefaction, and

gasification can be both thermal and hydrothermal.

Gasification of biomass occurs by partial oxidation of the carbon in the

biomass at high temperature (500-1300 °C) using a controlled amount of

oxidant that can be air, pure oxygen, or steam [38]. Depending on the oxidant,

the heating value of the produced gas can be 4-7 MJ/m3 (air) or 10-18 MJ/m3

(pure oxygen or steam) [23], [39]. In comparison, the energy value of natural

gas on average is 35-39 MJ/m3. Different types of biomass can be converted

into syngas through gasification. Syngas, depending on the feedstock, mainly

contains hydrogen, carbon monoxide, carbon dioxide, and methane, which can

20

be used to generate heat or electricity in an engine or turbine [23]. Gasification

can have a few challenging issues, including its impurities such as fly ash

particles and tar. Sulphur compounds, hydrogen chloride, alkaline

compounds, and ammonia are other possible impurities in syngas, which needs

to be purified before it is used [23]. For instance, the condensation of tar can

cause clogging or blockage of pipes, filters, or engines [23].

Pyrolysis can be separated into two main stages, solid devolatilization

(pyrolysis) and char conversion (combustion and gasification). The yields of

pyrolysis products, which are solid char, bio-oil (liquid), and gases such as

methane, depend on temperature, pressure, heating rate, and the type of

biomass [40], [41]. Pyrolysis can occur within the temperature range 300-600

°C while the operating temperature for pyrolysis on an industrial scale is often

more than 450 °C [38], [41].

Carbonization covers the thermochemical processes, at temperatures of

500-900 °C in an oxygen-starved environment, by which the carbon content of

biomass is increased [38]. The process results in a product, referred to as

charcoal. Torrefaction is a similar process to carbonization except that the

operating temperatures are at 200-300 °C while the biomass is without or only

little contact with oxygen [38]. The solid char made through torrefaction is

often referred to as biochar [42].

21

Combustion of biomass is the process of setting the biomass on fire and

making use of the heat, which represents the oldest way of biomass utilized by

civilization. The mentioned thermochemical processes face a common

challenge that is the need for a dry biomass. Furthermore, these processes

cannot eliminate ash and sulphur from the biomass [42].

Hydrothermal processes consist mainly of a wet environment; hence the

name hydro, and heat, which can help break down the components of the

biomass. Therefore, hydrothermal processes can use feedstock that is already

wet. Hydrothermal processes occur in a closed system where a reactor,

containing the liquid and feedstock, is heated, causing the pressure to increase.

One of the reasons for using water for the process is to use the catalytic

behaviours of water at sub and supercritical temperatures [1]. The water can

help hydrolyse and break the components of the biomass such as cellulose and

hemicellulose. Hydrothermal processes are separated into three different

categories depending on the temperature, as shown in Figure 2-5. These

processes can eliminate tar, and have higher yields compared with pyrolysis

and gasification [36]. Hydrothermal Gasification and Liquefaction occur in

supercritical water conditions, as shown in Figure 2-5, whereas hydrothermal

carbonization (HTC) operates at temperatures as low as 180 °C [1], [42].

22

Figure 2-5: Different hydrothermal processes separated according to their required temperature

range on a water phase diagram. The major products of each process are written in parentheses. The

figure is an adaptation from the work of Kambo et al. [42].

Hydrothermal Gasification (HTG) results in the production of gases

such as methane, hydrogen, and carbon dioxide after the feedstock is heated to

temperatures above 350 °C [43]. Hydrothermal Liquefaction (HTL) can

convert the biomass into liquid fuels using temperatures of 250-374 °C and

pressures of 4 to 22 MPa [36].

Contrary to HTG and HTL, hydrothermal carbonization (HTC) is a

process where the biomass is converted mainly into a solid material, referred

to as hydrochar [42]. The process is operated at temperatures above 180 °C for

durations of one to 72 hours under pressure so that the water stays in liquid

form [1]. Similar to other hydrothermal processes, HTC can also eliminate the

impurities of the biomass (i.e., extractives), lower its ash content, and increase

the energy content of the hydrochar product [44], [45].

23

2.3 Hydrothermal Carbonization (HTC)

Hydrothermal carbonization (HTC) is one of the hydrothermal

processes, which has recently received much attention from industry

and researchers [26]. HTC can occur at lower temperatures (180 – 300

°C) compared with HTL (300 – 374 °C) and HTG (which usually occurs

at temperatures above 374 °C), as shown in Figure 2-5, which leads to

lower energy consumption for the conversion of biomass. HTC produces

hydrochar, valuable chemicals, and by-product gases [1], as shown in

Figure 2-6. The products of HTC are 2-5% gas, 5-25% liquid, and 45-

70% solid depending on process conditions, such as temperature,

residence time, and solid load [37], [42]. The solid load is also known as

the liquid:biomass (L:B) ratio for the process.

Figure 2-6: Products of hydrothermal carbonization (HTC) separated according to their phases (i.e.,

gas, liquid, hydrochar solid) [1].

24

The type of feedstock can change the mass yields, process liquid

characteristics, and gases produced through the HTC process [1]. Some types

of biomass that have been processed with HTC include loblolly pine, tobacco

stalks, pine wood, algae, cow manure, paper, corn stalks, and miscanthus [42],

[46], [47].

Biomass goes through a series of reactions that cause the concentration

of oxygen and hydrogen to drop in the solid fraction while increasing the

concentrations of carbon [42]. These chemical reactions release other organic

chemicals such as acetic acid, levulinic acid, and 5-hydroxymethylfurfural

(HMF), which can be considered as value-added chemical products of the

process [5], [8], [45]. The gas is mainly made up of CO2, CH4, and H2 that can

be used as fuel [43].

2.3.1 Underlying Reactions

HTC involves a series of reactions that convert different components of

the biomass into hydrochar in addition to organic substances dissolved in the

liquid and some gas by-products, as shown in Figure 2-6. These reactions

25

include hydrolysis, dehydration, decarboxylation, condensation

polymerization, and aromatization.

HTC starts with hydrolysis of the biomass components that will result

in the production of ether and ester bonds, in the form of oligosaccharides [37],

[48]. Hydrolysis of cellulose, breakdown of this compound due to reaction with

water, happens when a water molecule splits into two ions of OH- and H+,

reacting with cellulose, in order to break down β1→4 bonds, as shown in Figure

2-7.

Figure 2-7: Hydrolysis of cellulose into oligosaccharides and glucose components [49].

26

Hydrolysis can eventually result in full decomposition of the physical

structure of the biomass into a liquid [37]. Acetic acid and glycolic acid are

formed after decomposition of hemicellulose, as summarized in Figure 2-8 [50].

During decomposition of cellulose and hemicellulose intermediate compounds

such as 5-hydroxymethylfurfural (HMF), furfural, and phenols are made.

These intermediate compounds can further polymerize and condense into solid

hydrochar, through which these compounds lose their OH- and H+ bonds [1],

[37]. Alternatively, the HMF can dehydrate back into levulinic acid and formic

acid, hence reducing the yield of the hydrochar [37], [51]. Decarboxylation also

occurs during hydrothermal carbonization, through a process which produces

CO2. Decarboxylation is when a chemical compound loses a carboxyl group

and CO2 is made. Formic acid is one of the compounds that goes through

decarboxylation and turns into CO2 and H2 [52]. Dehydration, which is the

removal of hydroxyl groups, results in carbonization of the biomass by lowering

the atomic ratios, O/C and H/C [37]. Dehydrated and decarboxylated

compounds, such as HMF, will then condense, polymerize, and form hydrochar.

The hydrochar polymer chains consist of stable aromatic bonds, which can be

considered as a basic building block of the hydrochar [1]. Aromaticity is used

to describe a cyclic (ring-shaped), planar (flat) molecule with a ring of

resonance bonds that exhibits more stability than other geometric or

connective arrangements with the same set of atoms. The liquid contains

27

nearly all the fragments from the decomposition of biomass components, which

favour the formation of hydrochar [37].

Figure 2-8: Chemical pathways for biomass components after decomposition [50].

2.3.2 HTC Products

Figure 2-9 shows different products of the HTC process as well as

potential applications of the hydrochar. Although HTC is commonly used to

produce an energy-dense solid hydrochar, the process liquid and the by-product

gas can potentially be used as a source for value-added products [42]. The

liquid contains chemicals, such as glycolic acid, formic acid, acetic acid,

levulinic acid, HMF, and furfural, as shown in Figure 2-8. Small amounts of

gas can also be extracted and used to generate heat for the process [37].

28

Figure 2-9: Products and possible applications of HTC [53].

2.3.2.1 Solid

Infrared spectroscopy of the hydrochar and the biomass is used to

explain the reactions and conversion mechanisms that the biomass goes

through during an HTC process [54]. The spectra of the biomass and

hydrochar, as shown in Figure 2-10, illustrate the peaks that correspond to a

particular type of chemical bond, such as OH (hydroxyl) bonds at ≈3400 cm-1

[55], [56]. IR spectroscopy of hydrochar shows aromatic bonds as well as

aliphatic (C—H at ≈2900 cm-1), and ether groups (C—O at ≈1100 cm-1) in the

product [53]. Dehydration of the hydrochar causes a reduction in the hydroxyl

groups [57]. C—O linkages at 1120−1050 cm-1 in the biomass, representing

ether bonds, decrease in intensity after the HTC process [53].

29

The peaks at 1600-1700 cm-1 suggest the formation of C=O bonds (part

of a carboxyl or carbonyl group), which can be formed after dehydration of

hydroxyl groups [53], [56].

Figure 2-10: Infrared spectra of initial biomass and final hydrochar after treatment at 240 °C.

30

Table 2-1 shows different functional groups and their corresponding

absorption wavenumbers in IR spectroscopy.

Table 2-1: Functional groups found in the biomass with their corresponding wavenumber. Data

gathered by Reza et al. [55].

Wavenumber (cm-1) Functional groups

3000-3600 OH stretching

2860-2970 C—Hn stretching

1700-1730

1510-1560

C=O stretching

1632 C=C

1613, 1450 C=C stretching

1430-1470 O—CH3

1400-1440 OH bending

1402 CH bending

1232 C—O—C stretching

1215 C—O stretching

1170, 1082 C—O—C stretching vibration

1108 OH association

1060 C—O stretching and deformation

700-900 C—H (cellulose, hemicellulose)

400-700 C—H (hemicellulose)

31

Van Krevelen diagrams can illustrate the change in the elemental

composition of the hydrochar compared with the biomass [42]. The diagram is

a tool to compare the hydrochar with the different types of solid fuels, such as

coal, lignite, and peat. Peat is a brown soil-like material, consisting mainly of

partly decomposed vegetable matter, while lignite is a soft brownish coal with

traces of plant structure, also referred to as brown coal. Lignite is formed from

naturally compressed peat. The hydrogen to carbon (H/C) and oxygen to

carbon (O/C) atomic ratios decrease while the carbon concentration increases

to values, that are close to peat, then lignite, then coal, and finally the best

carbon solid fuel, anthracite on a Van Krevelen diagram, Figure 2-11.

Figure 2-11: Van Krevelen diagram of HTC hydrochar produced with different conditions and from

different biomass types [42].

32

Figure 2-11 shows the atomic ratios (H/C and O/C) of different

hydrochars that have been produced at temperatures of 175-375 °C, for 30

minutes to 10 hours, from biomass feedstocks such as eucalyptus sawdust, corn

stover, birch, and coconut fibre [42]. Elemental analysis of the solid hydrochar

has shown an increase in the carbon content of the hydrochar compared with

the biomass, which is consistent with the findings of IR spectroscopy, i.e., an

increase in carbon-carbon bonds.

The hydrochar samples in Figure 2-11 had higher heating values

(HHVs) of 20-36 MJ/kg. The HHV is dependent upon the residence time,

temperature, and biomass used for the HTC process. The closer the hydrochar

is to the coal and anthracite regions in Figure 2-11, the higher the HHV [37].

HTC causes the release of inorganic minerals (ash) that do not contribute to

combustion, such as Si, K, Na, S, Cl, P, Ca, Mg, and Fe, into the process liquid

[25]. This reduction in the ash content, can increase the combustion

temperature and reduce fouling (in the form of solid salts, when these minerals

are oxidized) [58], [59].

The hydrochar is washed after extraction from the process to remove the

organic compounds that might be on its surface. The HHVs of organic

chemicals, which are formed during the HTC process, are less than the

hydrochar (e.g., HMF has an HHV value of 22 MJ/kg compared with 24-26

33

MJ/kg for hydrochar) [42]. Hydrochar is usually densified into pellets or

briquettes to simplify storage and transportation [55]. Densification, requires

the hydrochar to be heated to its glass transition temperature and compressed

into the shape of a pellet [60]. Heating to the hydrochar's glass transition

temperature permits enhanced molecular mobility (or plasticity) which

permits the hydrochar solid to soft and deform under pressure without

chemically decomposing. The glass transition of the hydrochar has been

reported to be 140 °C [61]. Densification conditions (i.e., pressure and

temperature) also depend on the shape of the pellets, which can be defined as

length to diameter ratio (L/D) for cylindrical pellets [61].

Densified pellets should be durable and hydrophobic [62]. Pellets with

higher durability produce less dust when transported. Durability is measured

according to ASTM D3402, which consists of rotating the pellets in a tumbler

at 24 rpm for 1400 revolutions. The ASTM D3402 is often adjusted for use on

a smaller scale in a lab by increasing the rotation speed of the tumbler or the

number of revolutions [63]–[65]. The pellet pieces left after the test are passed

through a 6.3 mm mesh and then weighed. The durability of the pellets will

then be calculated and represented as a Pellet Durability Index (PDI) or an

abrasion index.

34

The PDI is calculated as:

PDI (%) =𝑀𝑓

𝑀𝑖× 100, (1)

where 𝑀𝑖 and 𝑀𝑓 are the initial and final weight of the pellets. Higher PDI

values indicate higher durability.

The abrasion index is calculated as:

Abrasion Index =𝑀𝑖−𝑀𝑓

𝑀𝑖× 100, (2)

Higher abrasion index will indicate lower durability. In addition to high

durability, the pellets should resist compressive forces caused by stacking piles

of pellets during transportation or storage. The compressive strength of the

pellets is often measured using a material testing system (MTS) compression

test, where a pellet gets crushed using a known force [61].

The hydrochar pellets are favoured when hydrophobic, since water

absorption can increase the volume of the pellets to the point where the pellets

lose their solid structure. In addition, the heat needed to evaporate the

moisture content is considered as wasted energy. Hydrophobicity of the pellets

has been measured using the equilibrium moisture content (EMC) method [6],

35

[66]. EMC measures the moisture absorbed by the pellet after being exposed

to humid environments. In extreme conditions, water will pour directly onto

the pellets (e.g. rain), which places the pellets in direct contact with liquid

water. One approach to determining the water absorptivity of the densified

hydrochar is to measure the weight of the pellet after it has been fully

immersed in water. Total immersion of the pellet in water coupled with weight

measurements have been previously performed by Pimchuai et al., and Kambo

et al. [67], [68]. This approach can also be used to demonstrate the quality of

the densification of the hydrochar. Although hydrochar is hydrophobic [6],

[69], the densification process can leave empty pores within the densified

hydrochar. These pores can be filled with water, which would increase the

weight and the volume of the pellet.

2.3.2.2 Liquid

The process liquid contains most of the heat put into the process to

increase the temperature. Therefore, hot recycling of the liquid has been

suggested to maximize the use of energy and to lessen the need for disposing

of the process liquid[5], [45]. It has also been shown that recycling the process

liquid can increase the concentrations of organic compounds that can be

extracted as VACs [5], [8], [45].

36

2.3.2.3 Gas

The gas produced during HTC mainly consists of CO2; however, CO,

CH4, H2, and CmHn can also be produced [37], [42]. Decarboxylation is one of

the reactions that occur during the hydrothermal process producing CO2 [42],

dependent upon the process temperature [43]. Higher temperatures produce

more gases; however, the concentration of CO, produced through

decarbonylation, decreases as those for CH4 and H2 increase [43].

2.3.3 Temperature

Temperature plays a significant role in the production of the hydrochar.

The three main components of lignocellulosic biomass (hemicellulose, lignin

and cellulose) start to disintegrate and hydrolyze at 180, 200, and 220 °C,

respectively [70]. Furthermore, the decomposition temperatures can change

depending on the pH of the process liquid [70]. Elevated temperatures also

enhance solvent properties of water enough for even dissolving non-polar

compounds. The viscosity of water can change by up to one order of magnitude

depending on the temperature (when considering water at 0 °C versus at 350

°C), which can then enhance the penetration of water into the pores of biomass

and increase its disintegration [1], [71].

37

2.3.4 Residence Time

HTC has been reportedly performed in durations ranging from 5

minutes to 1 hour and then to several days [1], [6], [45]. The residence time

for HTC has shown to have a positive effect on the mass yield of the hydrochar,

which can be explained by further polymerization of already dissolved

compounds in the liquid. Long residence times can further disintegrate the

biomass and also the hydrochar already formed and put it back into the liquid

thereby lowering the mass yield which is not favorable for HTC process.

Therefore, optimizing the residence time for desired quality of the hydrochar

is imperative [37]. Increasing the operating temperature of the HTC process

can shorten the time needed for hydrochar formation [70], while temperatures

above 300 °C can result in liquefaction of the biomass, refer to Figure 2-5.

2.3.5 Pressure

HTC of biomass is operated at temperatures of 180 °C and above, under

pressure in a closed environment. The pressure can be slightly higher than

the saturation vapour pressure of water due to the production of gases such as

CO2 and H2 [1], [72]. Some authors have reported that increasing the pressure

at the operating temperature can increase the carbon content of the hydrochar

while Funke and Ziegler concluded that the small reported differences cannot

38

justify the increased carbon content to be caused only by the extra pressure [1].

Addition of LiCl to the process liquid has been found to decrease the operating

pressure of HTC, which can minimize the capital costs required to build the

reactor needed for the process [73].

2.3.6 Solid Load

The solid load, also referred to as liquid:biomass (L:B) ratio represents

the amount of liquid used for the HTC process with respect to the solid

biomass. The biomass is mixed into the process liquid (the medium in which

the chemical reactions take place). The process liquid (i.e., water) behaves as

both a reactant and a solvent. Increasing the L:B ratio (solid load) has been

suggested to have an adverse effect on the mass yield of the process [74], [75].

Higher amounts of liquid (i.e., higher L:B ratios) cause higher entropy, which

will result in chemical reactions to be driven towards where the entropy is

larger, therefore higher concentrations of the chemical compounds can be

found in the liquid, produced due to further disintegration of the biomass [76].

These compounds stay in the process liquid rather than polymerizing into

hydrochar, which would cause the mass yield to decrease. Optimizing the solid

load can increase the mass yield and the concentration of VACs in the process

liquid [76].

39

2.3.7 pH

Production of different organic acids during the HTC process has been

reported to cause the pH to decrease afterwards [45]. Lower pH has been

suggested to increase the rate of hydrolysis, which facilitates the

decomposition of the biomass components [2], [73], [77]. The pH also affects

the elemental composition of the hydrochar (i.e., C, O, and H concentrations).

Higher pH values are associated with H/C atomic ratios similar to that for

bituminous coal, while lower pH values result in reduced O/C ratios [1], [77].

Overall, using weak acids in HTC can improve the rate of reactions in the

process, which results in faster production of and higher HHVs for the

hydrochar [70], [78]. Ghanim et al. found sulphuric acid increased the mass

yield to ≈38% and the HHV to ≈23.8 MJ/kg, with pH of 2 for the process liquid

prior to HTC of poultry litter, compared with 27% and 21.9 MJ/kg, respectively

with an initial pH of ≈7 [77]. Meanwhile, alkaline conditions have been shown

to reduce the heavy metal concentration of sewage sludge after the HTC

process [3].

2.4 Catalysed Hydrothermal Carbonization (CHTC)

Catalysed hydrothermal carbonization (CHTC) is similar to HTC in

nature with a main difference that CHTC uses an acidic catalyst with pKa in

40

the range of 1.5-3.8 [4]. The lower pH enhances the decomposition of the

biomass components. Hence, hydrochar can be produced at lower temperatures

and shorter times [70]. The process can also be used to produce, potentially,

higher concentrations of organic compounds such as HMF, levulinic acid, and

formic acid [4].

41

Chapter 3: Catalysed Hydrothermal Carbonization with Process

Liquid Recycling

(Submitted and under revision)

Amin Ghaziaskar1+, Glenn A. McRae1++, Alexis Mackintosh2, Onita D. Basu3*

1 Department of Mechanical and Aerospace Engineering, Carleton University,

Canada, K1S 5B6

+amin.ghaziaskar@carleton.ca, ++glenn.mcrae@carleton.ca

2 PCS Technologies Inc., Vancouver, Canada

alexis@pcsbiofuels.com

3 Department of Civil and Environmental Engineering, Carleton University,

Canada, K1S 5B6

onita.basu@carleton.ca

Abstract

Catalysed Hydrothermal Carbonization (CHTC) was used to produce

hydrochar biofuel from wood chips at 240 °C in one-hour batches that included

recycling of the process liquid. Infrared spectra showed changes in the

chemical structure consistent with dehydration and decarboxylation. The

* Onita.Basu@carleton.ca, Tel: 613-520-2600, 1125 Colonel By Drive, Ottawa,

ON, Canada K1S 5B6

42

CHTC hydrochar had Higher Heating Values (HHV) of 28.3 MJ/kg, energy

yields of 64%, and H/C and O/C ratios similar to coal. The same process

without the catalyst (HTC) produced a hydrochar with HHV of 27 MJ/kg,

energy yield of 57%, and H/C and O/C ratios similar to lignite. Partial recycling

of the CHTC process liquid resulted in a 5% increase in the energy yield;

elemental composition, HHV, and scanning electron microscopic images of the

CHTC hydrochar for different recycles were indistinguishable. Densified

CHTC hydrochar pellets were 97% durable, and hydrophobic when compared

with wood pellets and torrefied wood pellets, which was shown by water

ingress measurements using an electrochemical cell with pellet electrodes. The

CHTC process with recycling has the potential to provide a green hydrochar

biofuel with excellent handling, storage and transportation properties that

could be a suitable direct replacement for coal.

Keywords:

Catalysed Hydrothermal Carbonization, Biomass, Hydrochar, Process liquid,

Recycling

Declaration of interest:

Alexis Mackintosh (co-author) holds patents on acid-catalysed hydrothermal

processes.

43

3.1 Introduction

Renewable fuels are being developed to supplement or replace coal to

mitigate climate change. Coal currently is used for 38% of global power

generation [79]. Replacing coal with solid biofuels would lower fossil carbon-

dioxide emissions.

Hydrothermal carbonization (HTC) is a process to produce a solid

carbon-neutral fuel called hydrochar, or biofuel. HTC hydrochar is deemed

carbon-neutral because the CO2 produced when it is burned is classified as part

of the natural or renewable carbon cycle [80]. Compared with coal, hydrochar

made from typical sources of biomass has low sulfur [81], which reduces the

formation of sulfur oxides and lessens acid rain impacts when burned for power

generation.

HTC makes hydrochar by the reaction of biomass in aqueous solution at

180-300 C. The process typically requires 1-72 h [1]; although shorter times

have been reported [45], [82]. Furthermore, because HTC is an aqueous

process, it is well suited to handling biomass with high moisture and may

decrease some process steps during production (i.e. drying).

44

The HTC process is a complex series of reactions including hydrolysis,

dehydration, decarbonylation, decarboxylation, polymerization, and re-

condensation [81]. These reactions will result in the production of gases,

liquid, and carbonaceous solid. The gas produced through HTC is mainly made

up of CO2, due to decarboxylation reactions [61]. The liquid of the HTC process

contains organic acids and intermediate compounds produced by degradation

of cellulose and lignin [5], [45]. The main components of the lignocellulosic

biomass (i.e., cellulose, hemicellulose, and lignin) react by hydrolysis forming

fragments such as glucose and fructose, which then further dehydrate into

intermediate compounds such as 5-hydroxymethylfurfural (HMF), furfural,

phenol, and monomeric sugars. The process reactions also produce organic

acids such as acetic acid, levulinic acid, and formic acid. These intermediate

compounds polymerize via acid-catalysed condensation into the hydrochar [1],

[48]. The carbonaceous solid (hydrochar) can have similar energy density as

coal. The organic acids produced during the process may also provide

additional commercial revenue if harvested.

The kinetics of the CHTC process is first order with an Arrhenius

activation energy of 152 kJ/mole associated with hydration of β(1→4) ether

bonds between glucose molecules in cellulose [83], [84]. To make hydrochar in

a reasonable time requires heat to bring the process liquid to the reaction

temperature. Hot recycling of the process liquid reduces energy costs by

lessening the heat required to raise subsequent batches to the reaction

45

temperature. Recycling also lessens the burden of waste management by

reducing the volume of process liquid to be treated to remove organic

substances. Additionally, the process liquid has shown catalytic properties that

increase dehydration reactions [5], and lower pH of the process liquid has been

found to improve properties of the hydrochar, including higher energy density

[2], [3], [73], [77], [85].

During the HTC process, the pH is lowered by the production of organic

acids, which enhance acid-catalysed condensation and dehydration reactions

and hydrochar production. These benefits of low pH accrue over time as the

process proceeds. These benefits might be realized sooner if low pH was

established at the start of the process rather than waiting for it to occur from

in-situ production of organic acids. The purpose of this study is to investigate

the benefits to the HTC process of controlling the pH to low values from the

start.

This study introduces Catalysed Hydrothermal Carbonization (CHTC)

in which a catalyst is added to the HTC process [4]. The catalyst is designed

to promote low pH reactions. This study presents the effects of the catalyst on

the HTC process, and results of low initial pH during recycling of the process

liquid. Chemical, physical, and mechanical characteristics of the hydrochar

reported include elemental composition, high heating value (HHV), mass yield,

46

surface properties, chemical bonds, compressibility, durability, and water

absorptivity.

3.2 Methods and Materials

3.2.1 Catalysed Hydrothermal Carbonization (CHTC)

CHTC experiments were conducted in 300 mL Parr bench-top reactors

(Parr Instrument Company, Moline). Approximately 10 g of woody biomass

was mixed into 120 mL of water and catalyst (liquid to biomass ratio of 12:1)

in a glass liner placed inside a reactor. The reactor was heated at 7 C/min

using a Parr rigid heating mantle (Parr Instrument Company, Moline) to 240

2.2 C where it was held one hour. The temperature was measured using a

type J thermocouple. The pressure was monitored using an analogue

transducer. After the process, the reactor air-cooled to room temperature and

gaseous products were vented in a fume hood to atmosphere.

The hydrochar was separated from the process liquid by vacuum

filtration with 0.45 m filter paper. Approximately 80% of the process liquid

was recovered after filtration. The pH of the process liquid was measured with

an ORION STAR A326 pH meter from Thermo Scientific (Dreieich, Germany).

47

The solid hydrochar was washed with 300 mL of distilled water and then dried

at 105 C for 24 h.

Recycling of the process liquid was conducted 15 times. Fresh catalyst

was added to recycled process liquid to makeup the reaction volume to 120 mL

and the initial pH 2.4. The hydrochar produced from the first process is labelled

“Fresh”, and the hydrochar samples from subsequent recycling experiments

are labelled sequentially R1-R15.

The same process was done but without the catalyst and without

recycling. The process, in this case, is simply labelled hydrothermal

carbonization, or HTC.

3.2.2 Materials

The feedstock biomass was wooden stir sticks (white pine and birch, 1

mm thick) that were ground to lengths 10 mm to 30 mm, and widths ≈2 mm,

and then stored in a closed container until processing. The moisture content of

48

the sticks was 4.5%. Torrefied wood and wood pellets were supplied by Airex

Energy (Laval, Québec, Canada).

3.2.3 Characterisation of the Solid Hydrochar

Mass yield was determined from the ratio of the masses of the hydrochar

and the feedstock, both on a dry mass basis.

The HHV of the feedstock and hydrochar was estimated in MJ/kg using

the unified correlation developed by Channiwala and Parikh [86]:

HHV = 0.3491𝐶 + 1.1783𝐻 + 0.1005𝑆 − 0.1034𝑂 − 0.0151𝑁 − 0.0211𝐴, (3-3)

where C, H, S, O, N, and A represent carbon, hydrogen, sulphur, oxygen,

nitrogen, and ash contents of the hydrochar, respectively, in mass percentages

on a dry basis.

The energy-densification ratio was calculated by dividing the HHV of

the hydrochar by the HHV of the biomass feedstock:

Energy densification ratio = HHV of the hydrochar

HHV of the biomass (3-4)

49

Energy yield was calculated as a product of the mass yield and the energy

densification ratio.

Moisture analysis was performed following ISO 18134 using a LECO

TGA 701 instrument (LECO Corporation, St. Joseph, MI). The carbon,

nitrogen, and hydrogen contents were determined with an Elementar Vario

Macro Cube (Elementar Analysensysteme GmbH, Langenselbold, Germany)

using ASTM D5373-16. Total sulfur was measured using an Elementar Trace

SN cube (Elementar Analysensysteme GmbH, Langenselbold, Germany)

according to ASTM 5453. The oxygen content was calculated by mass

difference (O% = 100 - (C% + H% + N%) - ash%). Ash content was measured

using a Mettler TGA/DSC1 using ASTM E1131-08. The elemental analyses

mentioned above for the feedstock biomass and hydrochar samples were done

by CanmetENERGY – Natural Resources Canada, Ottawa, ON. Infrared (IR)

spectra of hydrochar-KBr pellets, made from 10 mg of hydrochar mixed with

300 mg of KBr at room temperature, were measured with an ABB Bomem

MB100 (ABB Bomem, Quebec, Canada).

The morphology and surfaces of the hydrochar were observed with a

JEOL JSM-7500F (JEOL, Japan) Field Emission Scanning Electron

50

Microscope (SEM). Samples were dispersed on a conductive carbon pad and

coated with Au to a thickness of 5 nm.

3.2.4 Densification

The hydrochar was densified using a single-pellet press with a diameter

of 8.5 mm. The hydrochar was in the press for 10 min at 140 C prior to

pressing to ensure temperature uniformity. Pellets were made with pressures

of 70 MPa, 95 MPa, 135 MPa and 155 MPa held for 30 seconds. The pellets

were pushed out of the press and left to air-cool to room temperature. All the

pellets had a diameter of 8.50 ± 0.05 mm measured with a caliper. The length-

to-diameter ratios (L/D) of the pellets ranged from 1.1 to 2.2. Pellet masses

ranged from 0.5 g to 1.3 g. Mass density was calculated by dividing the mass

by the volume of the pellets. The energy density (MJ/m3) was calculated by

multiplying the mass density (kg/m3) and HHV (MJ/kg).

3.2.5 Mechanical Strength of Hydrochar Pellets

3.2.5.1 Durability Test

The durability of hydrochar pellets was quantified with the pellet

durability index (% PDI) using ASTM D3402, Standard Test Method for

Tumbler Test for Coke. This method consists of tumbling pellets in a can and

51

measuring the mass loss of the pellets. The metal tumbler had length 230 mm,

width 210 mm, and depth 90 mm. Two pellets from each recycle experiment

were tumbled for 10 min at 54 rpm with 600 wooden dowels (L/D=2). The

pellets were screened using a 2.38 mm sieve. The PDI was calculated using:

PDI (%) =𝑀𝑓

𝑀𝑖× 100, (3-5)

where 𝑀𝑖 and 𝑀𝑓 are the initial and final masses of the pellets. Results of the

current version of the method were compared with reported PDI values for

torrefied wood pellets from Airex Energy (Québec, Canada) and found to agree

within error of the value advertised by the producer (≥96%), which provides

confidence in the current numbers.

3.2.5.2 Compression Test

The compression strength of the pellets was determined using an

Instron 5582 testing machine (Instron, Norwood, MA). The pellets were placed

vertically on a cylindrical rod and compressed at a rate of 0.2 mm/min. The

force of compression and the displacement were recorded. The maximum force

a pellet could withstand before cracking or breaking was used to calculate the

maximum compressive strength of the pellet: i.e., by dividing the maximum

force by the surface area of the cross-section of the pellet.

52

3.2.5.3 Electrochemical water ingress (EWI) test

Water absorption of the densified hydrochar pellets was determined by

mass gain and electric current measurements. Hydrochar pellets were

partially immersed to a depth of 6 mm in electrolyte along with a carbon

electrode that served to create an electrochemical cell. A Solartron

Electrochemical Interface SI1287 (Solartron Analytical, UK) was used to

generate a potential difference of 0.5 V and the current passing through the

cell and pellet was measured for 24 hours. The voltage was chosen to prevent

electrolysis of the liquid. The electrolyte was deionized water with 10 g/L of

NaCl. Each pellet was weighed before and after the test with a Sartorius BP

2215 analytical balance with a precision of 0.1 mg. The water uptake was

calculated:

water uptake (%) = 𝑚𝑓−𝑚𝑖

𝑚𝑖× 100%, (3-4)

where 𝑚𝑖 was the initial mass of the pellet and 𝑚𝑓 the final mass of the pellet.

3.3 Results and Discussion

The first ‘Fresh’ processing was done with pure catalyst solution. The

pH of the recovered process liquid for subsequent recycle experiments was

between 2.65 and 2.8, as shown in Figure 3-1. Make-up catalyst solution was

used to standardize the initial pH of the recycled process liquid at 2.40 ± 0.07:

53

≈40% by volume was recycled process liquid and the rest was fresh make-up

catalyst solution. The higher pH of the process liquid effluent at the end of

each recycle, ≈2.7, is consistent with dilution caused by the water produced

from dehydration reactions.

Figure 3-1: Percentage ratio of make-up catalyst solution (Catalyst) to recovered process liquid

(Process liquid) used in 15 recycle experiments (stacked columns), circles show pH of the liquid

biomass mixture prior to start of the process (initial pH), and diamonds show pH of the process liquid

after CHTC (final pH).

54

3.3.1 Characterisation of the Solid Hydrochar

3.3.1.1 Influence of Process Liquid Recycling on Atomic Ratios (H/C-O/C) of the

Hydrochar

The goal of the CHTC process was to produce hydrochar that can be used

as a one-for-one replacement for coal, but better than coal because it will have

low sulfur and low ash. The similarity of the hydrochar and coal can be

illustrated with a Van Krevelen diagram and the hydrogen to carbon (H/C) and

oxygen to carbon (O/C) atomic ratios, Figure 3-2. Arrows in Figure 3-2 show

the effects of dehydration and decarboxylation reactions, which occur with

other reactions during the hydrothermal carbonization process.

Figure 3-2 shows that the ratios of H/C and O/C for the biomass (1.44,

0.66, respectively) are larger than the same ratios for the CHTC hydrochar

(0.79, 0.25, respectively). The CHTC hydrochar falls into the range of values

associated with coal. By contrast, the HTC process, which was done without

catalyst, but otherwise identical conditions, produced hydrochar with H/C and

O/C ratios 0.89 and 0.29, respectively, that are more like lignin, as shown by

the blue square in Figure 3-2. Similar ratios reported for hydrochar produced

from HTC using as feedstock biomass poplar wood (Stemann et al., 2013) [5]

and paper (Weiner et al., 2014) [8], processed at 220 °C and 200 °C,

respectively, and loblolly pine (Hoekman et al., 2013) [87] and tobacco stalks

55

(Cai et al., 2016) [88] which were both treated at 240 °C are also illustrated in

Figure 3-2. The O/C and H/C ratios of the hydrochar from HTC are positioned

in ranges associated with lignite (at best) and peat (at worst). The current

results showing lower O/C and H/C ratios for the CHTC hydrochar compared

with the HTC hydrochar suggest that the catalyst enhances dehydration and

decarboxylation, which is seen by the farther movement to the left and down

from the initial biomass region in Figure 3-2.

Elemental compositions of the feedstock biomass and hydrochar

produced from the partial recycle experiments can be found in Table 3-1 as

mass percentages. The carbon contents of the biomass were increased from

≈50% to 71.5% in the hydrochar while the concentration of the oxygen reduced

from 43.9% to 23.7%. The amounts of carbon, hydrogen, nitrogen, and oxygen

in the CHTC hydrochar did not vary significantly over the 15 recycles, Table

3-1.

After the CHTC process, the ash content in the hydrochar is half the

value in the biomass feedstock; 0.54% compared with 1.1%. This decrease in

ash is expected because the ash is composed mainly of inorganic cations that

naturally partition to the process liquid. Mixed results have been reported

regarding ash content and accumulation within recycled processes [5], [7], [45].

56

Table 3-1 Elemental composition (on dry basis, % mass) of the biomass and hydrochar, energy content,

energy densification ratio, and energy yield, (uncertainties in measurements are in units of least

significant figures unless otherwise noted). 1

Raw

biomas

s

HT

C

Fres

h

R1 R3 R4 R8 R12 R13 R15

C (%) 49.9 68.2 71.4 71 71.

5

71.

4

71 71.2 71.4 71.1

H (%) 5.98 5.06 4.71 4.9

8

4.9

0

4.9

0

4.9

0

4.87 4.73 4.88

N (%) 0.21 0.16 0.20 0.2

2

0.2

1

0.2

0

0.1

8

0.21 0.16 0.20

O (%) 43.9 26.6 23.7 24.

8

23.

4

23.

5

25.

1

23.7 23.7 23.8

HHV

(MJ/kg

)

19.9

±0.3

27.0

±0.4

28.0

±0.4

28.

2

±0.

9

28.

3

±0.

4

28.

3

±0.

4

28.

1

±1

28.1

±0.4

28.1

±0.4

28.1

±0.4

EDR2 1 1.36 1.41 1.4

1

1.4

2

1.4

2

1.4

1

1.41 1.41 1.41

EY3

(%)

57 64 68 69 67 68 66 68 69

1 Sulfur was less than 0.01% for all hydrochar samples

2 Energy densification ratio

3 Energy yield

57

Figure 3-2: Van Krevelen diagram showing atomic ratios of the feedstock biomass and the hydrochar

samples. The dehydration and decarboxylation reactions are enhanced by the catalyst producing

CHTC hydrochar with H/C and O/C ratios that fall in the region where lignite and coal overlap. The

H/C and O/C ratios of the hydrochar do not vary significantly with recycling. The hydrochar produced

without catalyst (HTC), but otherwise identical processing conditions, falls within the lignite region of

the diagram, shown by the blue square.

3.3.1.2 IR Spectroscopy of the Hydrochar

Infrared spectra can be used to track changes in bonding that occur

during the CHTC process. Figure 3-3 shows IR spectra of the feedstock

58

biomass, the hydrochar processed using only the catalyst solution (Fresh), and

the hydrochar after 15 partial recycles (R15), and HTC hydrochar produced in

this study (HTC). Chemical bonds found in woody biomass include –OH, C—

H, and C—O, which are absorbed at 3400, 2900, and 1100 cm-1, respectively

[56].

The intensity of the hydroxyl group peak, at 3400 cm-1, decreased after

CHTC of the woody biomass, which suggests a decrease in the number of OH

bonds consistent with the enhanced dehydration inferred from the lower O/C

and H/C ratios shown in Figure 3-2. Reduction of the number of hydroxyl

bonds is beneficial because it leads to an increase in hydrophobicity of the

hydrochar [57], which is desired when outside storage of hydrochar might be

subject to weather including rain. The intensity of the hydroxyl peak also

decreased following the HTC process of woody biomass, but not as much as it

did for CHTC. These results are consistent with lower O/C and H/C ratios for

CHTC hydrochar compared with HTC hydrochar shown in Figure 3-2. In

addition, the C—H bonds (found in aliphatic groups) at 2900 cm-1 [89] reduced

in intensity after CHTC, which is also consistent with dehydration of the

feedstock biomass. The intensity of C—O bonds at 1060 cm-1 found in

cellulose and hemicellulose in the form of alcohol, [53] were greatly reduced

after processing. However, the peak at 1200 cm-1, which also represents C—O

bonds, in the form of ether, was intensified in the hydrochar, showing the

59

transition occurring from alcohols into stronger ether bonds through CHTC

process.

The IR spectra also suggests that the CHTC process makes new

chemical bonds such as C=O, C=C, C—O found at 1700, 1600, and 1200 cm-1,

respectively. The C=O bonds can be found in the form of a carboxyl and/or a

carbonyl group [53], while C=C bonds in polymeric chains of the hydrochar.

Evidence of C—O bonds, which can also be found in lignin [44], increased after

CHTC.

Figure 3-3 shows that the chemical features inferred from the IR spectra

of the hydrochar samples (Fresh, and R15 are shown for comparison) were

relatively similar, but different from those in the raw biomass feedstock, which

suggests that chemical features of the hydrochar produced from recycling the

CHTC process liquid did not change appreciably.

60

Figure 3-3: IR spectra of raw biomass, Fresh, and R15. The spectra of the hydrochars were similar,

but different when compared with the spectra of the feedstock biomass.

61

3.3.1.3 SEM Images of the Hydrochar

SEM images are shown in Figure 3-4 for the woody biomass, and

hydrochar produced from the Fresh catalyst solution, the R1 recycle, and R15

recycle. Figure 3-4a and b show fibrous structure in the feedstock biomass.

After CHTC process, this fibrous structure disappears and is replaced with a

spherical and porous hydrochar product. Figure 3-4c, d, e, and h show the

hydrochar composed of spheres suggesting condensation about nucleation

sites. Figure 3-4g and f show the porous structure of the hydrochar. The images

of the hydrochar were indistinguishable regardless of recycling, which further

supports the consistency of the hydrochar produced from recycling the process

liquid. The porosity of the hydrochar is attributed to vaporization of volatile

components [90].

62

Figure 3-4: SEM images: (a, b) feedstock biomass showing fibrous structure; (c, d) spherical structure

of the hydrochar from fresh catalyst solution (Fresh); (e, f) porous structure of the hydrochar R1; (g,

h) R15 showing spherical and porous structure. The images of the hydrochar were indistinguishable,

but different from the feedstock biomass.

63

3.3.1.4 Mass Yields and Energy Values

The mass yields for the recycled samples are shown in Figure 3-5. The

mass yield of hydrochar produced with Fresh catalyst solution was 45%, but

for subsequent recycling, the mass yield was greater and constant at (48.3 ±

0.3)% (95% confidence).

Figure 3-5: Higher Heating Value (HHV), mass and energy yield of the hydrochar samples. The

mass yield increased by 2.7 percentage points for R1 compared with the initial process, which was

done using only the catalyst solution as the starting liquid phase (Fresh). Energy yield increased from

64% (Fresh) to an average of (68.6 ± 1)% for R1 to R15. The HHV is essentially unchanged for all

the recycles.

64

Hemicellulose and cellulose decompose hydrothermally at a useful rate

at 180 °C and 220 °C, respectively [1], into sugar monomers such as glucose,

fructose, and xylose that further degrade into intermediates such as formic

acid, acetic acid, levulinic acid, and HMF [48]; lignin decomposes around

200 °C into aromatic alcohols. These intermediate compounds condense and

polymerize into hydrochar. The mass yield from the initial process with fresh

catalyst was 45.5%. The mass yield increased to (48.3 ± 0.3)% for recycles R1-

R15. This increase is attributed to the hydrochar produced from the

intermediate compounds already in the process liquid from previous recycles.

Similar increases are seen for the energy yield: Pearson correlation coefficient

of 0.96, p=0.0002, for the energy yield and the volume of effluent added from

the previous processes.

The CHTC hydrochar made using only Fresh catalyst solution had an

energy content of 28.1 0.4 MJ/kg, which is an increase of 40% compared with

the HHV value of the feedstock biomass, 19.9 0.3 MJ/kg. The increased HHV

is attributed to the reduction of low-energy bonds from catalysed degradation

of the biomass cellulose, hemicellulose, and lignin, and formation of high-

energy bonds from polymerization of HMF into hydrochar. The HHV for CHTC

hydrochar using recycled process liquid was the same, 28.3 0.3 MJ/kg,

independent of recycling, as shown in Figure 3-5. The energy yield, equal to

65

the product of the mass yield and HHV, was likewise independent of recycling,

Figure 3-5.

The determined values of HHV, mass yield and energy yield of CHTC

hydrochar were all greater than the values obtained for HTC hydrochar. The

HHV values were 28.1 0.4 MJ/kg for the CHTC compared with 27.0 ± 0.4

MJ/kg for HTC hydrochar – similar HTC studies with similar woody biomass

feedstocks found (24 – 26.7) MJ/kg [6], [45], [87]. The mass yield of the CHTC

hydrochar was 45% compared with 42.3% for HTC. The overall performance

of the processes can be characterized with the energy yield, which was 64% for

CHTC and 57% for HTC. By these metrics, the CHTC process is superior to

the uncatalysed HTC process.

3.3.2 Densification

The pressure required to make pellets of woody biomass and hydrochar

depends on material properties, temperature, moisture content, and particle

size [91]. The hydrochar was pelletized at 140 °C, where the hydrochar

particles are expected to go through their glass transition [61], for the

pressures shown in Figure 3-6. At the glass temperature, the hydrochar

particles soften into a viscous state allowing flow into voids when pressure is

applied. At higher pressures the density of pellets was increased from 830 ±

66

50 kg/m3 at 70 MPa to 1050 ± 60 kg/m3 at 135 MPa, and 1070 ± 30 kg/m3 at 155

MPa. The density of the pellets was not found to vary with the number of times

the process liquid was recycled.

Figure 3-6: Average density and energy density of pellets made at four different pressures. The density

did not change after increasing the compression from 135 MPa to 155 MPa. The energy density reached

30 GJ/m3 at a density of 1070 ± 30 kg/m3.

3.3.3 Mechanical Properties of Hydrochar Pellets

Densified hydrochar pellets were subjected to durability, compression,

and water absorptivity tests to investigate their behaviour under extreme

67

service conditions. The hydrochar will likely be transported in large containers

where pellets will be stacked in piles, so pellets must have good compression

strength, and should be non-friable to avoid dust and potential breathing and

explosion hazards. These pellets should also withstand high humidity

including rain and water immersion.

3.3.3.1 Durability Test

Pellets made from feedstock biomass, and hydrochar from recycle R1,

R4, R6, R11, R13, and R15 were tested for durability, Table 3-2. The durability

of the hydrochar pellets were within a range of 96% to 97.1%, independent of

recycling the process liquid, except R11, which had durability of 94.5%. The

lower durability of R11 was likely because of lower densification pressure.

68

Table 3-2 Durability of the hydrochar pellets with pressure. Durability estimates were similar for

pellets made with pressures ≥ 135 MPa, but dropped by ≈2% when the pressure was reduced to 70

MPa.

Densification pressure

(MPa)

Sample Durability

155

R4 96.7%

R15 96.2%

135

R1 97.1%

R6 96.0%

R13 96.5%

70 R11 94.5%

3.3.3.2 Compression Test

Table 3-3 shows the compressive strengths determined for pellets made

from hydrochar produced in different recycles at best reached 14 MPa. Pellets

made from R2 and R10 hydrochar showed similar compressive strengths,

which suggests no effect of recycling.

More research is needed to determine the effects of moisture content,

particle size, and densification pressures on pelleting of CHTC hydrochar. In

the current study, densification pressures were less than or equal to 135 MPa

69

and compressive strengths were ≤ 14 MPa. Higher compressive strengths of

100 MPa for HTC hydrochar pelletized at 550 MPa have been reported [61].

Table 3-3 shows that the compressive strength of the pellets was lower for

pellets made with lower densification pressures, which suggests that higher

pressures could produce stronger pellets, if required.

Table 3-3: Compressive strength of densified hydrochar pellets with densification pressure. The

densification pressure was found to affect the compressive strength of the pellets while recycling the

process liquid did not.

Densification pressure (MPa) Sample

Maximum Compressive Strength

(MPa)

135

R2 14

R10 14

95 R9 9

70 R7 3

3.3.3.3 Electrochemical water ingress (EWI) test

Figure 3-7 shows the time-dependence of electric current passing

through an electrochemical cell composed of a sodium chloride solution with

one electrode a carbon rod and the other electrode a pellet made of wood,

hydrochar, or torrefied wood. Electrolyte ingress or penetration into the pellet

70

short-circuits the electrical resistance of the pellet which causes the current in

the cell to rise. Thus, current indicates water ingress.

The wood pellet disintegrated after 8 minutes in the electrochemical cell

as shown in Figure 3-7 by the vertical ‘spike’ reaching 16 mA. This

disintegration was mainly caused by the hydration forces (repulsive forces

acting in between surfaces separated by a thin layer of water) in between the

wood particles as a result of water absorption by cellulose, hemicellulose, and

lignin [92]. The current returned to zero when the wood pellet disappeared

into solution, Figure 3-8. The electrochemical cell with a torrefied-wood-pellet

electrode took 24 hours for the current to reach half the value measured

through the wood pellet when the wood pellet disintegrated. In contrast to

both the wood and the torrefied-wood pellets, the densified hydrochar pellets

showed more resistance against water ingress. The current in the

electrochemical cell was zero for the 24 hours tests with hydrochar pellets

made from pure catalyst solution (Fresh) and recycle R3. Tests with hydrochar

pellets from R8 and R14 showed very slight currents, less than 0.4 mA. The

hydrochar pellets (i.e., Fresh, R3, R8, and R14) gained 20% mass after 24

hours, while the volume increased by 5%, which suggests water ingress into

≈15% porosity.

Curiously, the hydrochar pellets absorbed mass, which presumably is

because of water ingress, but their resistance did not change, suggesting that

71

the water was only absorbed by a part of the pellets. The lack of current

increase also suggests reduced capillary motion of the water through the

hydrochar pellet, which is consistent with the reduced number of OH bonds in

the hydrochar inferred from the IR spectra, Figure 3-3. These bonds can help

elevate water through the porous structure of the pellet. The torrefied wood

and wood pellets have relatively more OH bonds which makes them

hydrophilic and prone to water absorption. Another curious feature is that air

bubbles are seen in Figure 3-8 on the surface of the hydrochar pellet, which is

also consistent with voids in the hydrochar pellets that can be filled with water.

Further experiments are needed to quantify water ingress into the pellet and

whether it is possible to reduce the number/volume of the voids and pores by

changing densification conditions.

72

Figure 3-7: Current through an electrochemical cell with pellet electrodes made of hydrochar, wood,

and torrefied wood. The wood disintegrated after a few minutes. Torrefied wood increased 42% in

mass after 24 h in the electrolyte. The small currents observed for hydrochar pellets suggests much

less water ingress.

Figure 3-8: Pictures of the pellets used in the EWI tests when they were first immersed in the

electrolyte (time=0) and later at the times indicated. The hydrochar pellets showed no visible

indications of breakdown products in the electrolyte, they maintained their shape and passed low

currents throughout the tests, as shown in Figure 3-9.

73

3.4 Conclusion

Catalysed Hydrothermal Carbonization (CHTC) of a woody biomass

feedstock in a catalysed solution has been shown to produce hydrochar with

hydrogen, oxygen and carbon contents that are similar to those of coal. In

contrast, the uncatalyzed control (HTC) sample produced a product similar to

lignite. The determined values of HHV, mass yield, and energy yield of CHTC

hydrochar were all greater than the values obtained for HTC hydrochar:

28.1 0.4 MJ/kg for CHTC compared with 27. 0 ± 0.4 MJ/kg for HTC; 45%

versus 42.3% for mass yields; and 64% versus 57% for energy yields. The ash

in the CHTC hydrochar was reduced to half the value in the feedstock.

Infrared spectroscopy showed the CHTC process reduces hydroxyl bonds and

creates more carbon-carbon bonds consistent with proposed condensation and

polymerization reactions. The hydrochar in SEM images appeared as porous

spheres. Process liquid could be recycled with no change in the HHV for the

hydrochar, but recycling increased the energy yield by 5%. Durability,

compressive strength, and water absorptivity of the densified hydrochar were

not influenced by recycling. Hydrochar pellets were hydrophobic compared

with wood pellets and torrefied wood pellets.

74

3.5 Acknowledgements

The authors would like to thank Professor Edward Lai of the Carleton

University and Guy Tourigny of CanmetEnergy for their help and support,

Neelesh Bhadwal of the University of Toronto for helpful discussions, and

Airex Energy for providing the torrefied pellets and wood pellets. The authors

would like to thank the Natural Sciences and Engineering Research Council

(NSERC) for helping fund this research.

75

Chapter 4: Production of Organic Compounds through Recycling

the Process Liquid of Catalysed Hydrothermal Carbonization

Planned submission.

Amin Ghaziaskara+, Glenn A. McRaea++, Alexis Mackintoshb, Edward P.C. Laic,

Onita Basud†

a Department of Mechanical and Aerospace Engineering, Carleton University,

Canada, K1S 5B6

+amin.ghaziaskar@carleton.ca, ++glenn.mcrae@carleton.ca

b PCS Technologies Inc., Vancouver, Canada

alexis@pcsbiofuels.com

c Department of Chemistry, Carleton University, Canada, K1S 5B6

edward.lai@carleton.ca

d Department of Civil and Environmental Engineering, Carleton University,

Canada, K1S 5B6

onita.basu@carleton.ca

† Onita.Basu@carleton.ca, Tel: 613-520-2600, 1125 Colonel By Drive, Ottawa,

ON, Canada K1S 5B6

76

Abstract

Catalysed hydrothermal carbonization (CHTC) of lignocellulosic

biomass has the potential to produce value-added compounds (VACs) from the

liquid effluent in addition to a solid hydrochar product. This research

examined the production of glycolic acid, formic acid, acetic acid, levulinic acid,

5-hydroxymethylfurfural, and furfural as potential VACs, in conjunction with

properties of the solid hydrochar such as higher heating value (HHV).

Recycling the process liquid increased the concentration of VACs and energy

yield of the hydrochar while the HHV stayed within the range 27.6 ± 0.3 MJ/kg.

liquid:biomass (L:B) ratio was found to be an effective way to increase the yield

of the VACs in both CHTC and HTC processes. The production yield of the

VACs and the mass yield of the hydrochar were negatively correlated after

changing the L:B ratios of CHTC and HTC. The research demonstrates the

potential to have multiple product streams from the CHTC process.

77

Keywords: Catalysed Hydrothermal Carbonization, Process liquid, Value-

added compounds, Recycling, Biomass

Declarations of interest:

Alexis Mackintosh (co-author) holds the patent on which the basis of this

research is conducted.

Graphical abstract

Highlights

• VACs increased in concentration after recycling the process liquid.

• Higher L:B ratios caused the yield of VACs to increase.

• Energy yield of CHTC increased, 72% to 80%, after recycling the

process liquid.

• Hydrochar had similar FC and VM to bituminous coal.

78

4.1 Introduction

Climate change is escalating as the use of fossil fuels continues [14]. The

contribution of these fuels (e.g., coal and oil) to greenhouse gas (GHG)

emissions has surpassed 30 GtCO2 eq/yr since 2010, which makes up over 60%

of the annual anthropogenic GHG emissions into the atmosphere [14]. The

increased use of coal compared with other energy sources has reversed the

decarbonization of the world’s energy supply [14]. Therefore, finding an energy

source replacement for coal-burning industries is imperative to reducing the

amount of GHG emissions into the atmosphere.

Hydrothermal carbonization (HTC) is a chemical process that produces

solid carbonaceous fuel, referred to as hydrochar, from a wide range of

renewable biomass (e.g., municipal waste, animal waste, forestry waste, and

agricultural waste). HTC involves the heating of the biomass in an aqueous

solution under elevated pressures to temperatures of at least 180 °C for times

on the scale of minutes to multiple days. HTC reduces the ash content in the

char compared with the biomass, increases surface area, and results in a

hydrophobic product that is easy to densify into a pellet when compared with

other treatment processes (e.g., torrefaction or pyrolysis) [58], [61].

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Catalysed hydrothermal carbonization (CHTC) of biomass is similar to

HTC but with an added catalyst to increase the rates of reactions occurring

throughout the process [4]. The hydrochar can be used as a one-to-one

replacement for coal because of its high energy content (≈28 MJ/kg) and similar

physical characteristics, but the hydrochar is better than coal because of its

lower ash contents (≈1% compared with 15.5% for bituminous coal) [93].

CHTC and HTC have the potential to produce various value-added

compounds (VACs) from biomass. The major components of lignocellulosic

biomass (i.e., cellulose, hemicellulose, and lignin) break into different

oligosaccharides, which subsequently hydrolyze into compounds such as

glucose/fructose. The sugar monomers can further dehydrate into

intermediate compounds (e.g. 5-hydroxymethylfurfural (HMF), furfural (FF),

and phenol) [76]. These intermediate compounds are more reactive and can

condense into hydrochar [48]. Furthermore, the hydrolysis of the biomass can

produce other compounds such as acetic acid (AA), glycolic acid (GA), levulinic

acid (LA), and formic acid (FA), which are considered as by-products and

cannot turn into hydrochar [94]. Alternatively, the HMF can dehydrate back

into LA and FA instead of condensing into hydrochar [95].

LA, HMF, and FF are of the most valuable chemicals that are produced

through hydrothermal carbonization of biomass [42]. The hydroxyl and

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aldehyde groups as well as furan rings in HMF and FF, and carbonyl and

carboxy groups in the LA, are convertible to a wide range of chemicals with

different industrial applications [96], [97]. FF is recognized as a platform

chemical for other furan based chemicals, such as furfuryl alcohol (FFA),

tetrahydrofurfuryl alcohol (THFA), and furoic acid, and can be produced

commercially for approximately 1100 $/ton [97], [98]. Industrial production of

HMF can be challenging; however, Kazi et al., 2011, suggested the price of 1050

$/ton, which can be highly dependent on the cost of feedstock. LA sold for 5000-

8000 $/ton in 2013 [96]. Other by-product VACs include GA, which is often

used in the production of cosmetics; AA, usually used as a solvent, and FA,

used as a pesticide for agricultural crops. Recycling the liquid phase has been

reported to increases the concentration of these VACs in the process liquid [5],

[45], and can be designed to preserve the energy used in a hydrothermal

process if the recycling is done hot.

Changing process variables of HTC (i.e., temperature, residence time,

and L:B ratio) has been found to affect the physicochemical propertise of the

hydrochar [75], [100]. In addition, altering L:B ratio for hydrothermal

carbonization of olive stones [76] and glucose [50] changed the concentrations

of HMF, FU, and LA in the process liquid.

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This paper examines the effects of recycling and L:B ratio on the

production of VACs and physicochemical properties of the hydrochar, in an

attempt to show the contribution of intermediate compounds, HMF and FF, to

hydrochar production, while other organic acids tend to saturate the process

liquid. Furthermore, the potential effects of L:B ratio on the quality and

rapidness of CHTC hydrolysis and dehydration reactions are studied.

4.2 Methods and Materials

4.2.1 Catalysed Hydrothermal Carbonization (CHTC)

CHTC experiments were conducted in 300 mL Parr bench-top reactors

(Parr Instrument Company, Moline). Approximately 10 g of woody biomass

was mixed into 50 mL of water and catalyst in a glass liner placed inside the

reactor, L:B ratio of 5:1. The reactor was heated at 7 C/min using a rigid

heating mantle up to 240 2.2 C where it was held constant for one hour. The

temperature was measured using a type J thermocouple and controlled with a

proportional–integral–derivative (PID) controller. The pressure was

monitored using an analog transducer. After the CHTC process, the reactor

was quenched cooled to room temperature and gaseous products were vented

into a fume hood.

The hydrochar was separated from the CHTC process liquid by vacuum

filtration through 0.45-m filter paper. Approximately 80% of the process

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liquid was recovered in the filtrate. The pH of the process liquid was measured

with a SP21 portable pH meter (VWR, USA). The solid hydrochar was washed

with 300 mL of distilled water and then dried at 105 C for 24 h.

Recycling of the CHTC process liquid was conducted 15 times. Fresh

catalyst (10-13 mL) was added to the process liquid to make up the 50-mL

reaction volume. The hydrochar produced from the first recycling process is

labeled “fresh”, and the hydrochar samples from subsequent recycling

processes were labeled R1-R15.

Similar CHTC process was carried out to examine different L:B ratios

while using the same amount of biomass (10 g). The volume of the liquid was

changed from 50 mL to 120, 100, and 30 mL which were referred to as 12:1,

10:1, and 3:1, L:B ratios, respectively. The HTC (non-catalysed) experiments

were carried out with deionized water without catalyst addition at the same

L:B ratios as the CHTC process; temperature and time of reaction were

conducted under the same conditions.

4.2.2 Materials

The feedstock biomass was wooden stir sticks (white pine and birch) that

were ground to 2.38 mm mesh particle size, and then stored in a closed

83

container until CHTC processing. The characteristics of the feedstock biomass

are summarized in Table 4-1.

Table 4-1 Higher heating value (HHV) and compositional analysis of the woody biomass on a dry basis.

Raw Biomass

HHV (MJ/kg) 19.6 ± 0.2

Total Volatile (%) 85.3 ± 0.2

Fixed Carbon (%) 13.5 ± 0.1

Ash (%) 1.1 ± 0.1

4.2.3 Characterization of the Process Liquid

Liquid chromatography was used for quantification of the known

compounds existing in the process liquid. All the analyses were done using a

Shimadzu Ultra-Fast Liquid Chromatograph (UFLC) (Shimadzu, Japan),

equipped with a DGU-20A5 degasser, a SIL-20AHT autosampler and an SPD-

M20A diode array detector (DAD). An LC-20A equipped with a gradient unit

was used to pump the mobile phase through the column. Windows-based Lab

Solutions software was used to control the UFLC system and to analyze the

chromatograms.

84

Phosphoric acid (88% pure), acetonitrile (HPLC grade), and acetic acid

(glacial) were purchased from Fisher Scientific. Furfural (98% pure) and 5-

hydroxymethylfurfural (95% pure) with the brand Alfa Aesar and formic acid

(99% pure) and levulinic acid (98% pure) with the brand Acros Organics were

purchased from Fisher Scientific. Glycolic acid (99% pure) was purchased from

Sigma Aldrich. Glycolic acid, formic acid, acetic acid, levulinic acid, 5-

hydroxymethylfurfural, and furfural are referred to as GA, FA, AA, LA, HMF,

and FF, respectively.

Chromatographic separation was achieved using a 150 mm 4.6 mm id,

5-m particle size Agilent Zorbax Sb-Aq column at 30 C with 10% of

acetonitrile and 90% of 76 mM phosphate buffer (pH=2) as the mobile phase at

a flow rate of 1.0 mL/min. All the organic compounds were detected at a

wavelength of 210 nm using the DAD detector.

After determining the concentration of each VAC, the yield was

calculated as a ratio of the total mass of the solute in process liquid to the dry

mass of the feedstock. Total mass of the solute was calculated as the product

of the concentration of the VAC and the liquid volume, which was 30, 50, 100,

and 120 mL for 3:1, 5:1, 10:1, and 12:1 L:B ratios, respectively.

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4.2.4 Characterization of the Solid Hydrochar

Mass yield was determined as a ratio of the hydrochar dry mass to the

feedstock dry mass.

Mass yield (%) = Hydrochar dry mass

Feedstock dry mass× 100 (6)

The Higher Heating Value (HHV) of the hydrochar samples were

measured using the Parr 6200 Isoperibol oxygen bomb calorimeter (Parr

Instrument, Moline) according to the American Standard Testing Method

(ASTM) D5865. Compositional analysis by thermogravimetry (i.e., moisture

content, volatile matter, fixed carbon, and ash) was performed according to

ASTM E1131 using a TGA/DSC 1 Thermogravimetric Analyzer & Differential

Scanning Calorimeter (Mettler, USA).

The energy-densification ratio was calculated by dividing the HHV of

the hydrochar by the HHV of the biomass feedstock:

Energy densification ratio = HHV of the hydrochar

HHV of the biomass (7)

Energy yield was calculated as a product of the mass yield and the

energy densification ratio.

Infrared (IR) spectra of hydrochar (10 mg) mixed with KBr (300 mg)

were measured with an MB100 spectrophotometer (ABB Bomem, Quebec,

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Canada). The morphology and surface of the hydrochar were observed with a

Vega-II XMU VPSEM Scanning Electron Microscope (SEM) (Tescan, Czech

Republic). Samples were dispersed on a conductive carbon pad and coated with

Au to a thickness of 7 nm.

4.3 Results and Discussion

4.3.1 Recycling the Process Liquid

During 15 repeated cycles of CHTC (at 240 °C with L:B ratio of 5:1), the

concentrations of the known VACs in the process liquid were determined. As

shown in Figure 4-1, AA was the most abundant VAC; its concentration

increased from 25.8 ± 0.7 g/L for the Fresh process liquid to 68.6 ± 1.9 g/L for

the last three recycles. The concentrations of GA and LA, similar to AA,

increased from 4.2 ± 0.3 g/L (Fresh) to 16.7 ± 0.5 g/L (R15) for GA and 10.4 ±

0.1 g/L (Fresh) to 16.9 ± 0.6 g/L for LA. Meanwhile, the concentration of HMF,

FF, and FA stayed within the ranges 3.9 ± 0.6 g/L, 5.7 ± 0.3 g/L, and 3.4 ± 0.6

g/L, respectively, throughout all recycles.

87

Figure 4-1: (a) VAC concentrations and (b) HHV, mass yield, and energy yield of the hydrochar

produced after recycling the CHTC process liquid, at 5:1 L:B ratio.

Figure 4-1a shows two general trends for the concentration of the VACs:

the increasing trend of AA, GA, and LA, and the almost constant

concentrations of HMF, FF, and FA. The HMF and FF are known as two of

the intermediate compounds, which can be transformed into hydrochar

through hydrothermal processes [94]. Near constant concentrations of the two

chemical compounds, HMF and FF, approve their contribution to the process

where they are consumed rather than accumulated. Furthermore, the

consumption of these compounds was illustrated by the increase seen in the

mass yield of the hydrochar, Figure 4-1b, after recycling the process liquid

effluent with initial concentrations of HMF and FF. [44], [101].

Chemical compounds such as AA, GA, and LA were suggested to have

low or no contribution to the production of hydrochar other than catalysing

hydrolysis and dehydration processes [101], [102]. Therefore, the

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concentration of the three VACs (i.e., AA, GA, and LA) was expected to increase

as the recycling of the process liquid continues. However, the increase in the

concentration of these chemicals in the process liquid slowed down, which

hinted approaching a saturation limit for the process liquid. Figure 4-2 shows

the accumulated concentrations of the VACs, AA, GA, and LA, which,

according to the fitted model, have reached their saturation limits.

FA is not known as a precursor to hydrochar and is unstable at

temperatures above 225 °C [1]; therefore, it goes through fast formation and

then decarboxylation reactions at 240 °C during the CHTC process. The near

constant concentration of the FA in the process liquid of the CHTC suggests

an equilibrium between the formation and decarboxylation of FA, which

ultimately results in the production of CO2, causing the operating pressure of

the CHTC to increase [1], [102].

89

Figure 4-2: The sum of AA, GA, and LA concentrations, represented as data and the fitted model.

As shown in Figure 4-1b, the mass yield increased from 51% (Fresh) to

55.8 ± 0.7 % (R2-R15). The HHV of the hydrochar was independent of recycling

the process liquid and stayed within the range 27.6 ± 0.3 MJ/kg, a 41% increase

compared with HHV of the biomass. Furthermore, the mass yield and the

HHV of the hydrochar (R2-R15) were negatively correlated with a Pearson

correlation coefficient of -0.78, p = 0.00023, as shown in Figure D-10, where

HHV was measured above average (27.6 MJ/kg) and the mass yield was below

its average value (55.8%). This correlation can suggest that the accumulated

VACs in the process liquid can limit the decomposition of cellulose and

hemicellulose into smaller monomers, which would result in higher mass

yields and lower HHVs compared with the initial cook (Fresh).

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Table 4-2 Mass yield, HHV, energy densification ratio, and compositional analysis (weight percentages

on dry basis) of the hydrochar after recycling the process liquid 15 times with L:B ratio of 5:1.

Mass yield

(%)

HHV

(MJ/kg) EDR1

Volatile Matter

(%)

Fixed Carbon

(%)

Ash

(%)

Fresh 50.6 27.7 ± 0.2 1.42 50.0 ± 0.3 48.8 ± 0.3 0.6

R1 53.7 27.4 ± 0.3 1.40 52.5 ± 0.2 46.4 ± 0.2 0.9

R2 55.5 27.6 ± 0.3 1.41 52.5 ± 0.2 46.8 ± 0.2 0.8

R3 55.4 27.7 ± 0.3 1.42 53.4 ± 0.2 46.4 ± 0.2 1.2

R4 54.8 28.1 ± 0.3 1.43 54.1 ± 0.3 46.6 ± 0.3 0.7

R5 55.8 27.4 ± 0.3 1.40 53.2 ± 0.2 46.1 ± 0.2 0.7

R6 56.4 27.8 ± 0.3 1.42 54.2 ± 0.6 46.6 ± 0.5 0.7

R7 56.4 27.5 ± 0.3 1.41 53.6 ± 0.2 45.5 ± 0.2 0.8

R8 56.3 27.4 ± 0.3 1.40 54.4 ± 0.2 45.4 ± 0.2 0.1

R9 56.3 27.3 ± 0.3 1.40 53.5 ± 0.2 45.7 ± 0.2 0.8

R10 56.6 27.5 ± 0.3 1.41 52.8 ± 0.2 46.1 ± 0.2 0.9

R11 54.9 28.0 ± 0.3 1.43 52.4 ± 0.2 46.7 ± 0.2 1.0

R12 54.6 27.9 ± 0.3 1.43 52.0 ± 0.2 46.9 ± 0.2 1.0

R13 55.9 27.5 ± 0.3 1.41 53.5 ± 0.2 45.9 ± 0.2 0.8

R14 56.9 27.1 ± 0.3 1.39 53.4 ± 0.2 46.1 ± 0.2 0.5

R15 55.6 28.0 ± 0.3 1.43 51.2 ± 0.2 47.9 ± 0.2 0.9

1Energy densification ratio

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The potential reduction in the decomposition of the biomass components

after recycling the process liquid can be further explained by the decrease in

fixed carbon (FC) content and increase in volatile matter (VM) compared with

the initial cook (Fresh), as shown in Table 4-2. VM and FC content of the

hydrochar slightly changed (2 percentage points) after recycling the process

liquid while independent of the number of recycles.

The FC content of the feedstock increased from 13.5% to 48.8%, which

is comparable to bituminous coal (approx. FC = 50.5%, VM = 34%, and ash =

15.5%, mass percentage on a dry basis) [93]. This increase makes the

hydrochar a good candidate for combustion instead of high-quality coals (e.g.,

bituminous coal). VM was reduced from 85.3% (raw biomass) to values within

the range 52.6 ± 1.1 % for the hydrochar. Higher VM in the hydrochar

compared with bituminous coal can suggest easier and faster combustion of the

hydrochar while similar FC contents can make the combustion as long-lasting,

resulting in high burning temperatures.

The ash content was measured at less than 1.2% for all the hydrochar

samples not resulting in a significant difference in between the biomass and

the hydrochar.

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The IR spectra of the hydrochar shows the changes to the chemical

structure of the biomass after processing it through CHTC, Figure C-1. The

IR spectra of the hydrochar were different from that for the biomass but

recycling the process liquid had no appreciable effects on the produced

hydrochar. One deviation between the hydrochar and the woody biomass was

a lower concentration of hydroxyl (OH) groups at 3400 cm-1 for the hydrochar.

Fewer OH groups are beneficial to the hydrochar since it results in higher

hydrophobicity [7]. The aliphatic group (C—H bonds) at ≈2900 cm-1 was

reduced in intensity for the hydrochars compared with the raw biomass. The

C=C bonds at ≈1600-1710 cm-1 representing lignin, cellulose, and aromatics

were more eminent in the hydrochar than in the biomass. The peak at ≈1050

cm-1, associated with C—O ether bonds and O—H stretching bonds (abundant

in woody biomass), had peaks with significantly less intensity in the IR spectra

of the hydrochars. The peaks at ≈600 cm-1, referring to C—H bonds in

hemicellulose [44], were weakened after CHTC process. The increase in the

carbon-carbon bonds, found within 400-1700 cm-1, was aligned with the

increased HHV and FC content of the hydrochar compared with the biomass.

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4.3.2 Effects of L:B Ratio on the VAC Concentrations and Hydrochar

Figure 4-3 shows the effects of changing L:B ratio of CHTC on the

concentration and production yields of the VACs. The production yield for all

the known VACs, except for the LA, was found to be the highest at 10:1 L:B

ratio. The concentration and production yield of LA further increased after

increasing the L:B ratio to 12:1 at 14.2 g/L with the yield of 18%. Increased

liquid volumes were in favor of LA production, through cellulose hydrolysis and

dehydration of HMF.

Biomass components decompose into smaller sugar monomers through

hydrolysis, which requires water [1]; therefore, larger volumes of water can

potentially increase the rate of hydrolysis in the CHTC. However, large

volumes of water require larger amounts of energy to heat the system to

desired temperatures (240 °C for this study). Decreasing the volume of the

process liquid can result in faster saturation of the process liquid, which either

negatively affects the performance of the process by limiting further

decomposition/hydrolysis of the biomass or, according to Le Chatelier's

principle, positively affects it by driving the reactions towards the hydrochar

formation.

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Higher production yields of the VACs after increasing L:B ratios, Figure

4-3b, were accompanied by lower mass yields of the hydrochar, as mentioned

in Table 4-3, suggesting better decomposition of the biomass components into

organic acids and intermediates while less condensation and polymerization of

the hydrochar. Larger volumes of process liquid require higher quantities of

the chemicals for the reactions to be pushed towards the formation of the

hydrochar. The sudden increase in the production yield of the LA and the

decrease in the mass yield of the hydrochar, for L:B ratios of 10:1 and 12:1,

both suggest that the dehydration of HMF into LA and FA was increased

rather than the formation of the hydrochar.

Figure 4-3: (a) Concentration of VACs in the process liquid after using different L:B ratios with CHTC

and (b) production yield of the VACs with respect to dry weight of the biomass used.

Meanwhile, changing the L:B ratio was found not to influence the HHV

of the hydrochar, Table 4-3. The HHV of the hydrochar stayed within the range

27.7 ± 0.1 MJ/kg for all L:B ratios. Furthermore, the energy yield increased

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from ≈63% to 75% after decreasing the L:B ratio from 12:1 to 3:1. The increase

in the energy yield of the hydrochar was mainly due to the 8% increase seen in

the mass yield of the hydrochar, refer to Table 4-3.

Table 4-3 Compositional analysis (dry basis weight percentages) of the hydrochar made with different

L:B ratios through CHTC process.

L:B ratio

3:1 5:1 10:1 12:1

Mass yield (%) 52.3 ± 0.3 50.2 ± 1.0 44.8 ± 0.1 44.6 ± 0.6

HHV (MJ/kg) 27.8 ± 0.2 27.7 ± 0.2 27.8 ± 0.1 27.6 ± 0.1

Energy yield (%) 74.5 ± 1.0 71.2 ± 1.9 64.0 ± 0.1 62.9 ± 1.0

Total Volatile (%) 52.1 ± 0.8 50 ± 0.3 49.5 ± 0.5 48.9 ± 0.1

Fixed Carbon (%) 47.3 ± 1.0 48.8 ± 0.3 49.6 ± 0.7 50.3 ± 0.1

Ash (%) 0.3 ± 0.2 0.6 ± 0.1 0.6 ± 0.1 0.7 ± 0.0

The ash content of the hydrochar did not change significantly, which

suggests the effects of L:B ratio on this factor to be minute. The slight change

in FC content and VM of the hydrochar may suggest that using larger process

liquid quantities results in an enhancement of the polymerization reactions by

producing more intermediate compounds through biomass hydrolysis and

other underlying reactions. The formation of the hydrochar can be more rapid

at lower L:B ratios when potentially shorter polymer chains are formed,

causing the 3% drop in FC content, as shown in Table 4-3.

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SEM images of the hydrochar, Figure C-3, illustrated the formation of the

polymerized particles in the form of microspheres on the leftover structure of

the biomass ( potentially the remaining cellulose and lignin [61]). The

hydrochar formation on the surface of the biomass can bring some

complications to further hydrolyzation of its components; the hydrochar is

known to be more hydrophobic than the biomass, which would cause the

hydrochar to repel water molecules, thus preventing further water ingress into

the biomass. This potential water deficiency can reduce the hydrolysis of the

biomass. Lower L:B ratios can further elevate this phenomenon; hence,

reducing the FC content of the hydrochar, Table 4-3.

The chemical structure of the hydrochar was found not to be affected by

changing L:B ratio of the process, Figure C-2. The hydroxyl group (≈3400 cm-

1) reduction in the hydrochar was eminent. The C-H bonds at ≈600 cm-1 in

cellulose and hemicellulose [55], were reduced after CHTC, which would occur

as a result of dehydration reactions throughout the process. Furthermore, the

C-O-C bonds at ≈1200 cm-1 were potentially formed after condensation of the

intermediate compounds into the hydrochar. Meanwhile, the C-O-C bonds can

be found in the cellulose as β-1,4-glycosidic bonds, ≈1050 cm-1 on the IR

spectrum, which break down through hydrolysis [103].

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Water ingress into biomass is vital for hydrolysis to occur; therefore, higher

L:B ratios can be preferable. Alternatively, CHTC can utilize lower L:B ratios

by using stirring mechanisms for the biomass and process liquid in order to

make sure of sufficient water ingression.

4.4 Conclusion

Recycling the process liquid of CHTC increased both the concentration of

VACs in the liquid and the energy yield of the hydrochar. Further

polymerization of the remaining intermediate compounds such as HMF and

furfural in the process liquid resulted in an increase in the mass yield of the

hydrochar while the HHV stayed within the range 27.6 ± 0.3 MJ/kg. Higher

concentrations of acetic, glycolic, and levulinic acid were suggested to

contribute less to the production of the hydrochar. Higher L:B ratios resulted

in higher VAC yields and lower mass yields, while the HHV of the hydrochar

stayed constant.

4.5 Acknowledgments

The authors would like to thank Margaret Anne Knowling of PCS

Biofuels Inc. for her help with the compositional analysis and bomb calorimetry

of the hydrochar.

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Chapter 5: Challenges and the Future

Catalysed hydrothermal carbonization (CHTC) was not found to

perform significantly better than HTC under the conditions mentioned in this

thesis, while lower pH of the process liquid has been shown to affect the

performance of the hydrothermal carbonization processes.

Determining the advantages of the hydrochar made through CHTC over

HTC require a more thorough look at the operating conditions of the CHTC

process compared with what was used for this study. Similar properties of the

hydrochar between the two processes were suspected to be caused by the excess

duration of the CHTC process (i.e., 60 minutes). If enough time is given for the

biomass to decompose and the hydrochar to polymerize, the use of catalyst can

be debatable, while shorter operating durations can potentially better

illustrate the advantages of the CHTC over HTC process caused by increasing

the rate of hydrolysis of the biomass components and condensation of the

hydrochar.

Future work can include a thorough look at the conditions used for the

CHTC process (i.e., temperature, time, concentration of the catalyst) while

considering different sources of feedstock in addition to the wood chips used for

this study. This will allow us to better understand the kinetics of the catalysed

99

hydrothermal carbonization (CHTC) of biomass and increase the efficiency of

the process by potentially decreasing the operating time and duration of the

process.

Economical consideration of the process is also at great importance

when the process liquid is recycled. Depending on the design of the reactor

(i.e., batch, binary batch, and continuous), recycled temperature, and mass, the

recovered energy from recycling varies. An economical simulation of CHTC

process should be instigated in the future.

The concentrations of the Valuable Added Compounds (VACs) in the

process liquid were also similar in between CHTC and HTC processes,

potentially caused by extended operating durations. These VACs have the

potential to be extracted from the process liquid of both CHTC and HTC

process in order to add value to these processes. Treatment methods and ways

to use the process liquid are also tasks worth exploring in the future.

100

Chapter 6: Conclusions

The objectives from the first chapter are listed below with the concluding

remarks:

I. Study the effects of partial recycling the CHTC process liquid on the

physical characteristics of the hydrochar.

The physical chemical properties of the hydrochar made through

Catalysed hydrothermal carbonization (CHTC) of wood chips were determined.

The hydrochar was produced by recycling a fraction of the effluent process

liquid with the addition of fresh catalyst, and a liquid:biomass (L:B) ratio of

12:1. Recycling the process liquid was done 15 times. The Higher Heating

Value (HHV) of the hydrochar samples stayed within the range 28.3 0.3

MJ/kg, while the mass yield increased from 45% when using fresh catalyst to

the range (48.3 ± 0.3) % for the recycles R1-R15. The hydrochar was shown to

have hydrogen, oxygen and carbon contents that were like those of coal. IR

spectroscopy showed the process reduces hydroxyl bonds and creates more

carbon-carbon bonds consistent with proposed condensation and

polymerization reactions. The hydrochar in SEM images appeared as porous

spheres. Process liquid could be recycled with no loss of hydrochar properties.

Durability, compressive strength, and water absorptivity of the densified

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hydrochar were not influenced by recycling. Hydrochar pellets were

hydrophobic compared with wood pellets and torrefied wood pellets. Water

ingress measurements were performed using an electrochemical cell with

pellet electrodes. Electrolyte ingress short-circuits the electrical resistance of

the pellet, which causes the current in the cell to rise. Thus, current indicates

water ingress. The hydrochar showed lower currents, while the current going

through the wood pellet and torrefied wood pellet increased, showing less

resistance against water ingress. The wood pellet disintegrated after 8

minutes; however, torrefied wood and hydrochar pellets resisted disintegration

for longer durations of 24 hours.

II. Study the effects of full recycling of the CHTC process liquid on the

chemical characteristics of the liquid effluent.

More than 80% of the process liquid effluent of CHTC was recycled for

15 times at the L:B ratio of 5:1. The HHV of the hydrochar samples stayed

within the range 27.6 ± 0.3 MJ/kg, while the mass yield increased from 50.6%

for the Fresh process to 55.8 ± 0.7 % for R2-R15 cooks. Through recycling the

process liquid, mass yield and HHV were found to be negatively correlated,

which suggested reduced decomposition of the biomass as a result of recycling

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the process liquid. The hydrochar showed similar fixed carbon (FC) contents

and volatile matter (VM) compared with bituminous coal.

The concentration of acetic acid, glycolic acid, and levulinic acid

saturated after recycling the process liquid. Meanwhile, the concentration of

the other valuable added compounds (VACs) i.e., HMF, furfural, and formic

acid stayed almost constant. The constant concentration of the formic acid was

suggested to have been caused by the formation of CO2 gases throughout the

process, while HMF and furfural were considered to be contributing to the

production of the hydrochar. Alternatively, the accumulation of acetic acid,

glycolic acid, and levulinic acid in the process liquid suggested that these

compounds contribute less to the production of the hydrochar compared with

the HMF and furfural.

III. Study the effects of L:B ratio on the hydrochar and process liquid

effluent of the CHTC process.

L:B ratio was shown to change the production yield of the VACs in a

CHTC process. The highest VAC production yields were for the L:B ratio of

10:1, except for levulinic acid, which had its highest production yield at L:B

ratio of 12:1. Increasing L:B ratio, though helpful for the production of VACs,

decreased the mass yield of the hydrochar. Higher L:B ratios helped to better

103

decompose the biomass and polymerize the hydrochar, which was suggested by

higher concentration of the VACs in the process liquid and the increase in the

FC content of the hydrochar, respectively. Alternatively, the HHV of the

hydrochar stayed within the range 27.8 ± 0.1 MJ/kg for all L:B ratios.

104

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Appendices

Appendix A : Contribution of the Authors

Article 1: Catalysed Hydrothermal Carbonization with Process Liquid

Recycling

Author 1: Amin Ghaziaskar

• Designed and performed the experiments

• Collected the data

• Performed data analysis

• Wrote the paper

Author 2: Dr. Glenn McRae

• Contributed to the design of experiments and analysis of the data

• Contributed to drafting and editing of the paper

Author 3: Alexis Mackintosh

• Contributed to drafting the paper

Author 4: Dr. Onita Basu

• Contributed to the design of experiments and analysis of the data

• Contributed to drafting and editing of the paper

120

Article 2: Production of Organic Compounds through Recycling the Process

Liquid of Catalysed Hydrothermal Carbonization

Author 1: Amin Ghaziaskar

• Designed and performed the experiments

• Collected the data

• Performed data analysis

• Wrote the paper

Author 2: Dr. Glenn McRae

• Contributed to the design of experiments and analysis of the data

• Contributed to drafting and editing of the paper

Author 3: Alexis Mackintosh

• Contributed to drafting the paper

Author 4: Dr. Edward P.C. Lai

• Contributed to drafting the paper

Author 5: Dr. Onita Basu

• Contributed to the design of experiments and analysis of the data

• Contributed to drafting and editing of the paper

121

Appendix B : Hydrothermal Carbonization of Woody Biomass

Hydrothermal carbonization (HTC) of woody biomass was done to

determine its differences from the CHTC process. Four different L:B ratios,

3:1, 5:1, 10:1, and 12:1 were used. The processes were done similar to CHTC

except using only deionized water instead of the catalyst. The hydrochar

produced through HTC had lower HHV values compared with the hydrochar

produced through CHTC; however, the differences were not significant. Table

A-1 shows the differences in between the hydrochar made through CHTC and

HTC.

Table B-1: Properties of the hydrochar made through CHTC and HTC processes.

Liquid:Biomass ratio

CHTC HTC

3:1 5:1 10:1 12:1 3:1 5:1 10:1 12:1

HHV (MJ/kg)

27.8 27.7 27.8 27.6 27.2 27.3 26.7 26.9

EDR1 1.42 1.42 1.43 1.41 1.40 1.40 1.37 1.36

EY2 (%) 74.5% 72.0% 64.0% 62.9% 71.5%

68.1% 59.9% 57.0%

VM3 (%) 52.1 ± 0.8

50 ± 0.3 49.5 ± 0.5

50 ± 0.1 51.0 ± 0.1

52.8 ± 0.1

54.7 ± 0.2

53.5 ± 0.3

FC4 (%) 47.3 ± 1 48.8 ± 0.3

49.6 ± 0.7

50.4 ± 0.1

49.0 ± 0.1

47.2 ± 0.2

44.7 ± 0.2

47.9 ± 0.3

Ash (%) 0.3 ± 0.2

0.6 ± 0.0

0.6 ± 0.1

0.0 ± 0.3

0.0 ± 0.1

0.0 ± 0.1

0.2 ± 0.0

0.0 ± 0.8

1 Energy Densification Ratio 2 Energy Yield 3 Volatile Matter 4 Fixed Carbon

122

The HTC process produced the Value-Added Compounds (VACs), which

were found in the process liquid of CHTC process. Figure A-1 shows the

concentration of these compounds in the process liquid as well as their

production yields.

Figure B-1: (a) The concentration of VACs after changing L:B ratios for HTC processes and (b) the

production yield of the VACs with respect to the biomass.

The properties of the hydrochar made through HTC was not significantly

different from its counterpart CHTC. The concentrations of the VACs were

similar, except the concentration of LA, compared with Figure 4-3.

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Figure B-2: IR spectra of the HTC hydrochar produced using different L:B ratios. The chemical

bonds were found to be similar to CHTC hydrochar while the difference in % transmittance values of

the CHTC hydrochar were slightly larger.

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Appendix C : Instrumentation

Figure C-1: CHTC reactor setup. The reactor is heated using a band heater and a hot plate. The

temperature is monitored and controlled using the PID controller. The pressure is only monitored

during the process.

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Figure C-2: A temperature profile sample of the CHTC process.

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Figure C-3: Single-pellet press assembly. The schematic was provided by Guy Tourigny from

Canmet Energy.

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Figure C-4: Single-pellet press assembly.

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Figure C-5: The Instron 5582 testing machine used to measure compression strength of the hydrochar

pellets.

Figure C-6: Test cell for determining the water ingress into the pellets. The pellet shown is a torrefied

wood pellet at t=0.

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Figure C-7: Shimadzu UFLC setup

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Appendix D : Supplemental Data to Chapter 4

D.1 IR Spectroscopy

The following are the IR spectra of the hydrochar made through recycling the

process liquid at the L:B ratio of 5:1, Figure C-1, and the IR spectra of

hydrochar made through CHTC with different L:B ratios, Figure C-2.

Figure D-1: IR spectra of raw biomass and hydrochar after recycling the process liquid.

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Figure D-2: IR spectra of biomass and the hydrochar made through CHTC using different L:B ratios.

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Figure D-3: ATR-IR spectra of hydrochar produced with different L:B ratios. The peaks represent

similar chemical bonds to the ones mentioned for FT-IR spectra.

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Figure D-3: ATR-IR spectra of the hydrochar after recycling the process liquid (R15), using pure

catalyst (Fresh), compared with the Raw biomass.

The IR spectra of the hydrochar has a slope in the range 4000-2000 cm-1

compared with the ATR-IR spectra, mainly caused by Rayleigh principle,

where the hydrochar particle sizes are on the order of the IR wavelengths.

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D.2 SEM Images of the Hydrochar and the Biomass

Figure D-4: SEM images taken from the biomass (wood) and the hydrochar for various L:B ratios.

The fibrous structure, mainly caused by lignin, was persistent while spherical hydrochar particles were

formed.

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Figure D-5: SEM images of hydrochar produced with CHTC at 5:1 L:B ratio. The hydrochar made

with fresh catalyst is Fresh and the hydrochar made after recycling the process liquid once and 15

times are called R1 and R15, respectively. The hydrochar was formed using the recycled process liquid.

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Figure C-5 shows the correlation in between the mass yield and the

energy yield, and lack of it in between the HHV and the energy yield, though

both HHV and mass yield are used to calculate the energy yield of the

hydrochar. Therefore, recycling the process liquid can improve the energy

yield of the hydrochar by increasing its mass yield.

Figure D-6: Energy yield vs mass yield (left) and HHV (right). Energy yield has a higher dependency

on the mass yield compared with the HHV. The energy yield of the CHTC process with 5:1 (L:B) ratio

was found to be independent of the HHV of the hydrochar and correlated with the mass yield with a

Pearson correlation coefficient of (0.93) with p = 4.05E-08.

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Figure D-7: Using more process liquid resulted in an increase in the mass yield. Lower volumes of

makeup catalyst resulted in higher mass yield percentages, which suggested further polymerization of

leftover chemical compounds in the process liquid.

Figure D-8: The initial concentration of the catalyst (catalyst) used as the process liquid (Fresh) and

after recycling the process liquid (R1-R15). The concentration of the catalyst decreased to ≈0.3 g/L

in the process liquid. The constant concentration of the catalyst after recycling can suggest the

possibility of reducing its concentration.

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Figure D-9: Volatile matter, fixed carbon content, and ash content of the hydrochar made through

CHTC with L:B ratio of 5:1.

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Figure D-10: The negative correlation in between the HHV and mass yield of the hydrochar from

recycle range R2-R15. The hydrochar was made through CHTC with L:B ratio of 5:1.

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Figure D-11: Changes made to fixed carbon content and volatile matter of the hydrochar after

changing the L:B ratio

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Figure D-12: HHV, mass yield, and energy yield of the hydrochar made through CHTC with different

L:B ratios.

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Appendix E : UFLC Calibration Curves

The calibration curves used for UFLC analysis are provided in this

section, Figures D-1 to D-6.

Figure E-1: Calibration curve of formic acid.

The process liquid samples run with UFLC were 20 times diluted prior

to the test and were each run at least in duplicates. A standard was also

injected along with the unknown to validate the calibration curves. The

concentrations represented for the compounds were averaged from the

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injection trials (at least two) of one process liquid sample. Integration of the

peaks in the spectra were all done by software, LC Shimadzu. Slope of the

baseline was autodetected for each spectrum.

Figure E-2: Calibration curve of acetic acid.

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Figure E-3: Calibration curve for levulinic acid.

Figure E-4: Calibration curve used for glycolic acid.

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Figure E-5: Calibration curve for HMF.

Figure E-6: Calibration curve for Furfural